Yeah,
I think I'm having the same issue - I do see the login screen, but the upper half of the screen is just black. Switching to console and back fixes that and I can log in, but then there' are random glitches on parts of the screen/windows, etc. Disabling compositor mostly fixed that, but not 100%.
How did you install the 5.8.16 kernel? When I do something like
$ qubes-dom0-update kernel-lateat-5.8.16
it shows me the package, downloads it, but then it tells me "no package kernel-latest-5.8.16 available" so I'm a bit confused.
I'm currently running on 4.19.155 which works, but I'm not sure it fully supports the new CPU in the laptop (mainly the power management).
regards