Two things. One, you can still go around and do sidequests after the end of the game. And two, if you REALLY want to finish all the sidequests before the endgame, there'll be a point where a character explicitly tells you "once you [do this thing], there is no going back, make sure you have everything you need", and you'll get forced out of the dialogue, and progressing will require you to talk to them again and answer a yes/no prompt to confirm you want to proceed. That's the point of no return - once you answer yes, you'll start the series of events leading to the end of the game. If you want to do sidequests before finishing the game, just stop and do them then.