Ok, I'm at the point where I can decide how to fight for the Hoover Dam (or choose not to yet).
The problem is, I still have at best 60% of the map locations to discover yet.
How can I be ready to end the game if there's a majority of the game yet to play?
Do I simply not choose to fight yet and just wander the wastes and do whatever quests I run into?
This is what I've been doing over the past few days.
Can I fight for the Dam and still roam and discover/complete quests or will I have to start a new game?

That's the point of no return, you can choose not to proceed and go explore and finish off sidequests/DLC.

And after the battle, the game ends. No free roam.