Been thinking a lot about how strong a psyker Heinrix actually is, plus what he could and couldn't realistically achieve with his biomancy and sanctic daemonology. The 40K psyker system is not exactly a hard magic system and very much subject to "you can do whatever fits what you are writing", but I like myself some guidelines at least for my own writing. So I did some research to get a hopefully realistic picture.
I am disregarding gameplay here (the build you give him and the insane damage numbers you can achieve are, in my opinion, at most dubiously canonical) but AM keeping in mind the kinds of threats the retinue + RT emerge victorious against ingame (even if a lot of those REALLY should have killed you if you didn't have the "named character in 40K" buff...)
Heinrix's Psychic Assignment
For those unaware, the Imperium ranks its psykers based on their level of psychic ability (or lack thereof). This rating goes from Omega (also referred to as Blanks or Pariahs - individuals with a negative presence in the warp that can dampen psychic activity around them) all the way to Omega/Omega+ (probably around where the Emperor resides).
We can already infer the approximate range Heinrix is in based on the fact that he was considered worth training. An individual is considered a psyker and taken by the Black Ships if their rating is at least Iota. However, weak and/or unstable psykers are not worth training in the Telepathica. Instead, these individuals are sacrificed to sustain the Astronomican. Especially due to his training to become a Primaris Psyker, we can thus assume that Heinrix was, at the time, at least above average; Theta or higher.
We can also assume that he was not especially powerful, however. While present day Heinrix possesses an extraordinary amount of willpower and I'm sure ~12 year old Heinrix was not exactly weak of mind, his first manifestation, comparatively speaking, was not very dramatic. While he killed someone, there was no collateral damage, at least not anything spoken of. No Warp breach, no psychic phenomena; just the accidental use of a power he would go on to use in the course of his work. Lastly, we can also assume he was only middlingly powerful because his family kept him alive long enough to hand him over. Black Ships don't show up every day, so unless he got very (un)lucky, it's likely that he would have been in confinement on his home planet for up to a year. The more powerful the psyker, the more unstable they are and the more daemons take an interest (the stronger the psyker, the stronger their presence in the Warp). If he had been actively unstable, I strongly doubt that he would not have been killed before a Black Ship arrived. He'd be a liability.
Because of this, I would put ~12 year old Heinrix at High Theta to low Eta. Not weak, but not extraordinarily powerful. However, his abilities as a biomancer and his notable willpower would have made him an attractive psyker to spend resources training.
An individual's classification can change over time. Considering his Telepathica training, his time in the Guard and his drcades of service in the Inquisition, I am comfortable putting present-day Heinrix at Zeta or even Epsilon level. The feats we see him accomplish ingame, such as being able to stand up against Chaos Space Marines and even a Patriarch (albeit with help), certainly speak to a far above average psyker, arguably with room to grow should he expand his powers of sanctic daemonology.
In my quest to figure out a concrete framework for his capabilities, I first looked to things Heinrix is canonically capable of ingame. Since depending on your interpretation, abilities you can give him through level-up (like super speed) may not be canonical feats, I have divided this section into subsections accordingly.
Please LMK if I forgot or missed anything!
Boiling blood (Backstory, introductory scene)
Related, he can apparently make people float/hover using his powers (introductory scene)
Mending/closing wounds (mentioned during the monastery)
Dulling/removing pain (mentioned during Commoragh, done on RT after falling down the elevator during the Void Shadows DLC)
Righting injuries such as bones pulled out of sockets (also elevator in Void Shadows)
Slowing or halting own aging (mentioned during bridge dialogue)
Reconstructing his own eye (backstory, but just how damaged it was (gone? Just nonfunctional?) isn't explicitly stated)
Fixing even severe nerve damage (backstory and Commoragh)
Regrowing bones and skin (backstory; the metal plate in the back of his head)
Iron Arm; reinforcing muscles
Regeneration on self + others
Poisoning others through attacks and psychic powers
Reducing others' speed and strength
Speeding himself and others up
Protecting (Psyker only?) RT from the psychic shrieks on Rykad Minoris
Word of the Emperor (mentioned in Bridge dialogue)
Mass heals or damage buff through Word of the Emperor
Ranged Immolate Soul damage that is especially effective against daemons
Summoning a flaming sanctic swords that is capable of producing fire
Resulting Limits and Capabilities
Naturally, these are just snapshots of what he can do. Still, based on this and based on how complicated certain processes would actually be to influence, below is my assessment of what Heinrix can and can't do.
Mending non-complicated wounds such as flesh wounds and uncomplicated breaks. Easy on himself and others.
Accellerating someone's metabolism or immune system, to promote wound healing, expelling poisons or killing infections. Easy on himself and others. This would likely sap a lot of the person's energy as their nody is being made to expend energy at a rapid rate.
Sensing his own and other's physical state. Determining if they are unwell would likely be trivial; determining what exactly is causing it would need more time.
Accelerating someone's heartbeat (basically how blood boil works).
Sensing the presence of the Warp or daemons.
Shielding his mind or other's minds against daemonic influence (the block may be harder to uphold against stronger daemons. It also gets harder to upkeep the more people he is shielding).
Obviously channeling power through his force sword.
Mending more complicated wounds such as internal bleeding or organ damage. Mending non-clean breaks. This not only requires understanding and locating the source of the damage, but also expending power pretty precisely in order to fix it. He can do it, but not in the middle of combat: He likely needs some time to focus. He can likely apply a temporary fix even during combat, however this fix would only be upheld through continuous use of his powers: A bandaid, not a true cure to the issue.
Repelling or banishing daemonic influence. This heavily depends on the daemon in question, but would likely require a considerable battle of wills.
Strengthening/reinforcing muscles. Especially as a split-second decision.
Super speed. Since this is considered biomancy, I assume he would be doing a lot of changes in people's leg strength. Far from trivial, especially if you consider that you would also need to be able to think faster to keep up with your own speed.
Killing bacterial infections. I doubt he could do anything against viral infections outside of aiding the immune system. However, with time and focus, he could likely find and kill bacteria in an organism. Finding the right ones is another story, but I think it's possible.
Any neural or nerve manipulation on himself. This is VERY precise work and it's honestly shocking and impressive he can do it at all. It likely requires a lot of focus and precision and a strong understanding of biology.
Undoing warp mutations. While possible, it'd likely once more require very precise knowledge of the area of the body and how all the muscles, blood vessels, organs, ... are meant to look and sit. This would likely take hours of focused work and take a lot out of him.
Banishing daemons that have entered into realspace.
Halting or slowing his own aging. Aging involves a ton of different processes; manipulating it likely requires constant upkeep. I believe he could also do it for others, but it'd require weekly, if not daily, maintenance sessions.
Changing his own or others' appearance. While ex. changing hair color would be easy, changing facial or body structure would be much more tricky.
Fixing wounds where ex. shrapnel is stuck in the body. Biomancy cannot remove the shrapnel; he'd need tools to do so before he could heal the person.
Massively altering someone's biology. I doubt he is powerful enough to actively mutate someone (without killing them). This includes extra limbs, organ fuckery, etc.
Brain damage. The brain is simply too complex. Even if he could reconstruct it, he could not reconstruct the affected memories or motor functions.
Reanimating a dead person. If regular medicine could nit bring them back, I doubt he could either. He may be able to temporarily (while he upkeeps the power) restart their processes, but if they are already braindead this would essentially be a zombie puppeteered by him.
So about the Sanctic Daemonology...
Bigger lore nerds can correct me, but to my understanding, sanctic powers are NOT innate in the way biomancy/telepathy/... are. They must be learned, and it is considerably difficult to do so. His sanctic powers should thus be far less advanced than his biomancy.
Also, from my understanding, psykers can learn powers outside of their specialty / natural aptitude. However, it is difficult to do so and depending on the source, it is only possible to learn easy powers of other disciplines. So Heinrix could (and likely would) pick up minor telepathy to help with interrogations, for example.
That was a lot of yapping, but I hope my thoughts were interesting and maybe helpful! At the end of the day, psykers can do anything the plot needs them to (especially if they push themselves) but this should be a fairly realistic assessment of what Heinrix would be capable of. Feel free to correct me though - I'm still figuring this out myself.