First: I have to apologize for something. I wrote here:
but there is one i in nvidia too much. It must be sudo apt install nvidia-driver-535 I have it changed.
But when the Driver is installed, it seemes that You have wrote it correctly. So, now the Driver works with the 6.5 Kernel but not with the 6.8 Kernel, yes? You could try it with the 550 Driver instead.
For this You have to remove the old Driver first with sudo apt remove nvidia* libnvidia* like before and then install the 550 Driver with sudo apt install nvidia-driver-550 (this Time are the right Numbers of i's).
Why it isn't in the List, I don't know. Maybe because it is a newer one. Not the newest, but newer than the one's in the List. Maybe Zorin have added the Driver; could be a Possibility, too.
Hmm ... You could try a newer Kernel but that don't guaranteed that it runs ... So, at this Moment ... When the 6.5 Kernel with the 535 Nvidia Driver works, You could use that for now. When the 6.8 Kernel gets Updates maybe it will work better in the Future.