Maybe it helps to set an environment variable for the gsk_renderer.
You could also try to add the kelebek333 ppa. I'm not sure if it works with kernel 6.8 or if you need kernel 5.15. In the ppa are given some instructions about a modulepath, maybe you need to add them?
But I'm not sure if you'll get an improvement. I personally went back to nouveau driver after testing the kelebek333 ppa with 390 driver.