so does anyone know if there's a way to check what chests I have/need to get in each world in KH FM? I kinda remember there being a way to but I haven't played this game in a while and I can't find it in the journal or anything
This feature isn't available in KH1 (or KHFM). It only started appearing in the series starting with KH2.
Yeah, I'd say backtracking and looking up wikias are your best bet. But I think there is a trophy to indicate you have opened all the chests...?
There's a trophy for opening 100 chests. So unless there are only 100 chests in the whole game, he won't have to open ever single one in the game for the trophy.
I'm not just opening all the chests for the trophy though, it's something I try to do in every KH game. I guess I'll try using a guide or something to find them.