The only thing that should be relevant to you is “Enable Southern Islands (SI) and Sea Islands (CIK) support” to use the older amdgpu driver (instead of radeon). Your card seems to be GCN 2.
You are using a Wayland WM, so you don’t need xserver settings. (Firefox on NixOS is already defaulting to Wayland, btw.)
HIP and OpenCL are for GPU compute, which you definitely don’t need.
Vulkan is already enabled by default (only 64 bit, not 32 bit, which is basically relevant only for games). It does not support the old radeon driver.