Hint: you may have better luck using vc4-kms-v3d than vc4- f kms-v3d with these kernels atm. What issues are you seeing with fkms? I thought I'd got to the bottom of the issue on fkms and it is working fine for me. Buster appears to have a broken Mesa at the moment though, so x is giving me issues....
why didn't update firmware still 4.14.94 ? I think you'll have to explain what you have done to get any help. Have you run rpi-update? What did it output? Have you rebooted? What does uname -a report? don't know does it call firmware or not? but saw it in github modules kernel bump to 4.19.23 20 ho...
in my case, almost switch to wayland/sway with VC4 driver drm-backend.
chromium, vlc-without-mmal-patch, ffmpeg-with-mmal-enable, sdl2-with-enable-rpi, etc.
everything went well for desktop using daily.
if want kodi need switch to X11, that's annoyed.