Kinda late, but I think the present time helps enlighten some aspects of the story, revealing facts and circumstances we weren't aware beforehand and having us better understand what happened in the past, as some users have said before. Essentially, if the present is scrapped in part 1, it simply becomes an afterthought, it kind of loses it’s importance, since the story wouldn’t need to progress past part 1.
One big issue about it is that in this case present characters may act unnatural or generic when interacting with the MC, so only specific (past) characters should interact with present MC in part 1 to avoid contradicting past events, as well as having more content in past than the present.
Regarding Julia, I think it would make more sense if MC and Julia were in fact friends, but with unexplored feelings for each other, with the trip being a way of MC getting closure on past events and considering entering a relationship with her, instead of proposing marriage. That way, Julia’s participation in the story would be more linear while still giving the player the option to pursue her or not. Besides, MC inviting Julia as a friend and leaving Sebastian behind would be kinda weird lol.
These are just my thoughts, though. Best of luck, Dev!