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.