You said you don't like Angel Exterminatus for the same reason you don't like The First Heretic. If I may, what is that reason?
Something of a problem I had with the BL series going all the way back to Horus Rising and A Thousand Sons, though it didn’t really come to the fore until TFH. Basically ‘well actually [character] wasn’t a bad person and never really wanted to do a war but he had a bad childhood and the Emperor is a bad father so he got angry and eventually Chaos happened because the Emperor is a bad’
Like, I get that the writers want complexity instead of having the Chaos guys just be bwa-ha-haing evulz from day one, I get that, and I fully support it. Over and over I’ve said that my favorite Chaos primarch stories are old-school Alpharius and Mortarion, both of which are fundamentally about their central primarchs failing to find common ground with the Emperor with both sides being a little at fault. In my opinion the BL series (especially ADB) puts too much of the failing on the Emperor, blaming him for seemingly all the bad things that ever happened so that their personal favorites can be pure and good and just became Chaos because they were pushed into it, or otherwise because just-as-planned.
To some extend I sort of understand - BL is so obsessed with grimdark that I suspect some of them want to paint the Imperium and that galaxy at large as being so fucked, so fundamentally broken that becoming a Chaos pawn is perversely somehow the ‘best’ choice. That doesn’t jive for me, though, because if you just decide that the Emperor was a jerkoff and a moron right from the beginning and everything was always grimdark then what’s even the point of the Horus Heresy anymore? The whole theme of the Horus Heresy is the fall from grace, the destruction of Eden and the exile from paradise - the original version of the tale even had Horus leading ‘a third’ of the Imperium’s armies against the Emperor, thus straight-up drawing the parallel between Horus and Lucifer. Remove that and instead of the story of the fall, you’re just going from one flavor of grimdark to a different, slightly less subtle flavor of grimdark.
I don’t expect the Emperor and the loyalists to be perfect pristine pure did-no-wrong-ever goody-two-shoes, but I prefer an interpretation of the Heresy where Emps is at least given a fair shake, creating an admittedly-flawed state in the belief that he could stabilize things over time, once humanity was united. Instead he’s treated as a derp who can’t people and threatens his gene-sons with execution and erasure if they fuck up.