Thanks for the pointer @Jerith 
I’m not fully in the know about the difference between ati_unfree vs amdgpu vs amdgpu-pro, apart from this: that amdgpu is open-source, ati_unfree is the closed source version for older cards and amdgpu-pro is the closed source version for the newer cards.
When amdgpu loaded earlier, it was able to detect my card and the log output seemed to indicate that this driver could work.
I found this while surfing and tried switching the channel to unstable and forcing boot.kernelPackages = pkgs.linuxPackages_5_4 as shared there and it works.
Thanks for all the help!