Thanks for accepting this bounty! Here you can talk with other bounty hunters as you progress through the game and keep everyone up to date on your progress.

If you beat the final boss, can you still go back and 100% obtain all the items? I'm at 90-something% in some areas and can't find the last items for the life of me.

I'm not sure on that, I'm definitely way behind where you are I don't recall being able to continue exploring after defeating the final boss in previous Metroid games, but I've only beaten a handful and it's been a while.

The answer is yes, you can go back to right before the final boss. (You'll definitely need all those missile tanks btw).