If I had to make up a backstory for Sentinel, it would be that before Archa 7, Sentinel would be a very quiet and serious guy. Like Michael Myers type way, he wouldn't say much and wouldn't even react. It's like a lifeless bot.
In my head, it would make sense that old SG Sentinel wouldn't meet SG Elita One but meet Optimus AFTER Archa 7, basically it would be Optimus be the one suggesting to take a trip to the planet without Sentinel there.
Because in this AU, SG Optimus is the reason to why Sentinel goes from dead and serious to seemingly cheery and alive. It's canon that SG Sentinel loves Optimus, so in basically, Optimus gave a meaning to SG Sentinel.
However, I could somewhat make Old SG Sentinel and SG Elita One meet but they wouldn't be close. Basically old SG Sentinel would be sharing dorms with SG Optimus and SG Elita One. But Elita One and Optimus wouldn't pay no mind to Sentinel since according to both, "He's a big loser"
There were times that SG Elita One would be alone with Sentinel and she would lowkey tried to get a reaction out of him but she would fail. Elita One wonder what was his problem. Old SG Sentinel was also uninterested in getting to know Elita One.
Now I said about Optimus be the one taking to Elita One to Archa 7 alone but I would insert that Sentinel is the reason that Optimus knows of Archa 7 since even though Sentinel didn't get excited about life, he would feel something towards Organics.
But I think it would make more sense if Optimus made a convincing deal that Sentinel should bring Optimus and Elita One along. Sentinel's original plan was to go alone to see a organic spider. Sentinel in the end agrees if Optimus and Elita One help.
The trio go to the planet, and Optimus waste no time in separating himself and Elita One from Sentinel. Because Optimus secret plan was to kill Elita One. Optimus and Elita relationship was abusive. They would often fight and argue. One time, Elita did something extraordinary in some training and she was admired by everyone. Optimus felt immense rage, he couldn't believe that Elita took the spotlight away from him.
You see Optimus would be the one to feel entitled that anyone should be grateful for him. That's the case with Elita One. Optimus wants her to believe she needs him. Despite the low-self esteem that Elita One would have, she would eventually get tired if being mistreated and looked down by Optimus constantly
After Elita One's popularity, Optimus made a snarky comment about her achievement which cause an ugly argument between the two. Elita defended herself and Optimus absolutely hated that. He realized from this now, Elita won't be under his thumb anymore. He wanted to make her pay and then heard Sentinel sneaking off to Archa 7.
After throwing Elita into a long pit, Optimus tries to find Sentinel and eventually does except he also found that this planet was an energon mining. Optimus is happily screaming while Sentinel doesn't comprehend the excitement. Sentinel then notices that Elita is missing b nlnknd asks Optimus about it. Optimus then stays quiet and pathetically lies that Elita left Optimus and he was trying to find her. Sentinel knows he's lying but he doesn't really care. Betrayal in the Autobot society isn't really surprising.
Sentinel then asks if Optimus had seen any spiders, Optimus replies with a no, this time he was telling the truth. Sentinel expression showed a bit of disappointment. Optimus then feels curious about this quiet bot. And starts asking him questions. Sentinel would reply but without enthusiasm. While Optimus was trying to make a plan in making a secret business to sell Energon, Sentinel was wandering around the planet to find any spiders, unfortunately as Optimus was trying to steal energon, an alarm went off. Optimus quickly realizes this place wasn't alone after all and runs away to find Sentinel. Thankfully Sentinel also went back to Optimus but before they could leave the place, they got busted by Autobot police.
Apparently, that abandoned energon mine was property of Ultra Magnus. You could say it's his secret stash. Both Optimus and Sentinel were questioned. Sentinel told the truth, he took Optimus and Elita to explore the planet but he didn't know of the energon mine, he mainly wanted to interact with a spider.
Optimus tries very hard to make up a good lie but fails. Ultra Magnus mostly punish Optimus about stealing the energon than the death of Elita One. Optimus is then put in the maintenance team while Sentinel being put under surveillance, Ultra can see some potential within Sentinel.
After the whole incident, it actually pulled Optimus and Sentinel more closer. You see Optimus felt intrigued towards Sentinel. So he would try to contact Sentinel. The plowmech would surprisingly interact back. At some point, Sentinel confided with Optimus that he would always feel empty and have suicidal thoughts which Optimus took advantage and gaslight Sentinel that he could fix him.
Fifty years later, Sentinel has totally changed. He's more expressive, everybot likes hanging out with him, and is always in a good mood. Sentinel and Optimus relationship is very one sided but Optimus treats Sentinel better than Elita, especially since Sentinel never fights back and even loves how Optimus controls him. Sentinel has given absolute loyalty to the maintenance bot and has grown a very strong love towards Optimus. Sentinel's purpose in life is to satisfy Optimus in his needs and wants. If Optimus wants Sentinel to become a weapon for him, Sentinel will gladly do it so.
Sentinel only became a high elite guard because Optimus wants him to keep tabs with the Magnus. Optimus wants to take down Magnus especially after being thrown away, and demands Sentinel to help him and the plowmech obliges.
While Elita finds her relationship with Optimus very traumatizing, Sentinel finds his relationship with Optimus very precious.
Optimus is very confident in keeping Sentinel under his thumb until a certain white spiderbot appears in the picture
SG Sentinel: Optimus, I love you so much!
SG Optimus: Bring me a can of oil
SG Sentinel: Do you also love me?
SG Optimus: I will if you bring me a can of oil