Fets will be better for voltage drop. But for high side switching in this design you definitely want p fets - they are basically a drop in replacement.

I don’t know if you need an lm317 replacement. Aside from the voltage drop issue, the only issue I’m aware of might be startup timing since you’re PWMing them. I could not find anything about this in a datasheet. Once you get the transistors figured out I would use an oscilloscope to probe your output and see if it is the waveform you want.

I do know there are multi-channel LED drivers that would work, and are definitely made for PWMing. So you have options if the lm317s are too slow.