This works very well in Steam, setting
__NV_PRIME_RENDER_OFFLOAD=1 __GLX_VENDOR_LIBRARY_NAME=nvidia %command%
in the game’s lauch options like you already did. I’m using this ever since render offload was working and I don’t think I’m the only one.
Maybe this is the same bug some users reported:
https://forums.developer.nvidia.com/t/performance-regression-on-prime-system-with-520-and-later-driver-series/241178
Did you try downgrading the driver?