https://wiki.archlinux.org/index.php/AMDGPU#AMDGPU_PRO
https://wiki.archlinux.org/index.php/AMDGPU_PRO
Do note that all of the AMDGPU-PRO packages are in AUR, not in the Manjaro Repo.
5.9 is EOL. Have you tried 5.10 or 5.11?
I have a 5700 XT and it works great on 5.10, didn’t work well on 5.8 or 5.9 though.