Can I finish side quests after completing the main story?
I'm three-quarters through the main story, and am wondering if I complete it will I still be able to finish the side quests?
Yes, according to users in threads like this one, completing the game will return you to a "pre-completed" state in which you are able to go complete whatever side quests you'd like.
It's not a point of no return, you can still play after the Finale, it just reverts you to a 'pre-Finale' state (similar to the base game)
oh ok, then the side missions will still be available to do?
In my experience, yes.
Here's another thread discussing this:
Can I finish the story mode first then do the side quests after?
Nothing is missable. Once you finish the story missions it sticks you back in free roam.