I wish the reason why Marty and Doc are friends in '85 was that they've met in '55, and so Doc befriends Marty later on so the whole loop works. But that's not how these movies were written.
Yeah, unfortunately that’s not how time travel works in the BttF universe. This sounds more Time-Turner/ Harry Potter/ predestination, which is a different but equally cool version of time travel to explore!
I like that BttF is more about “our actions affect things” rather than “it doesn’t matter because it’s all preordained.” And maybe, to some extent, BttF has elements of both, but I sadly don’t think the premise of the story even works if it’s a matter of predestination. It’s all about the impact of our choices — that we make a difference, even with our smallest actions.