Replay or keep playing after finishing the plot?

I rushed the story a bit and feel like I didn't take my time in between missions to properly appreciate the shifting dynamic, characters' moods, changes in the world and such. And for that reason I want to replay the whole game. Knowing the characters and the plot I think I'll be able to notice more nuance while taking my time between major plot points.

But the most important things for me is to find all the random interactions and odd locations. Is it all still there after finishing the main plot or do available strangers and side missions change over time?