Are there any videos, tutorials, or even forum posts that goes into detail how to remove the diffuser (without having to remove the bumper)?

 

There's always the Toyota Technical Information System: https://techinfo.toyota.com/

I think you'll find what you're looking for under Repair Manuals.

I got the $15 / 2-day access once when I was installing my amp/sub. Well worth it.

 

You'll for sure need to remove the bumper, I tried doing it recently without removing the bumper and it was the biggest headache. There's this screw right behind the reflector that you'll need some kind of thing ratchet screwdriver to get to if you want to not remove the bumper. The working space is literally 2 inches