From my experiences on Arch and Manjaro from a Linux gamer's perspective, the only thing from AMDGPU-Pro we might want to consider is getting AMDGPU-Pro's AMDVLK implementation and using that...though, for me lately (Mesa 20+), it has performed worse than RADV across the board with my RX 580. AMDVLK-Pro can live beside RADV and the open source AMDVLK so it's pretty safe to install and forget about.
I recommend installing it from TK-Glitch's PKGBUILD collection.
I do not recommend installing the complete AMDGPU-Pro driver on Manjaro (or Arch). I've never had good results installing all of it. Nothing in it will really benefit your 590 or my 580. Figure I'll mention that AMDGPU-Pro's OpenCL package can be installed alone as well. That and AMDVLK-Pro are the only things from AMDGPU-Pro you should consider using on Manjaro. The OpenCL package isn't something needed for gaming.
The only people that actually require AMDGPU-Pro are people who buy bleeding-edge hardware since it can take a kernel release or two for what's in AMD's staging repos to make it to mainline and AMD can sidestep that a bit with Pro packages.
In that AMD thread you mentioned using Mesa-git -- is that still the case? A complete AMDGPU-Pro + Mesa-git likely will not work together outside of the two packages I mentioned above (and I use them with Mesa-git).
It's really worth mentioning that there has also been a lot of changes to Mesa-git between 19.2, 19.3 and 20.0 and that it wouldn't hurt to revert back to what the repositories offer to ensure you didn't have a bad build. There's also a mesa-git PKGBUILD at the same git repo as the AMDVLK-Pro one I linked to above. It's what I use for Mesa-git.
Also, what game or games in particular are you having issues with?
EDIT: After reading your AMD post a bit more....I think your 590 overheats like my 580 did before I undervolted it. I could hardly game before it would either get really laggy or just conk out. After watching the thermals and seeing that it would thermal throttle and de-throttle over and over again, I undervolted my timings and got it to where it stayed below 80C (and overclocked my memory to boot).
I used WattmanGTK to view temps and tweak settings and then created a systemd unit to load my undervolt settings on boot. Been doing that for almost a year now and it's the literal first thing I do upon a fresh Linux install because I can't play games w/o it regardless of the distribution.