As others said, every room & system is different. There are no cookie cutter rules. I personally do the following:
Diffusers in the front and back wall. Depending on the distance from the back wall, the impact they have there is bigger. My diffusers are actually combination diffuser absorption and bass trap (GIK Alpha 6” and 4”)
Absorbers and diffusers (a mix of them two) in side walls and ceiling. Lots of experimentation needed. Actual work. Listening and REW measurements. There is no free lunch.
Base traps in both front and back corners.
In my opinion and experience, the coverage area is important. You cannot just slap a few panels here and there and call it a day.
‘Good luck. Lots of work but rewarding at the end