I have two machines running Zorin 16.3 Pro, and have yet to upgrade to Zorin 17.2 Pro, mainly because I require the 5.15.x kernel to utilize legacy proprietary video drivers which are required to run certain games with any sort of playability (i.e. without horrid FPS).
I have attempted in the past to do a full 17.2 install (not upgrade), and rollback the kernel from 6.8.x to 5.15.x, with disastrous results... requiring me to just reinstall 16.3. But seeing as Zorin 16.3 is EOL this April, I have only a couple of months to come up with a solution (or just run without security patches, which I'd rather not do).
Is it possible to lock the kernel to 5.15.x before upgrading and keep the proprietary video drivers in place?
I would use this method to set the default kernel prior to running the upgrader to upgrade to 17:
Before booting into 17, I would then boot into the Recovery menu and ensure that the Grub File is set to as you set it above, not reverted to default.
I would then ensure that the 5.15 kernel was installed.
I would move back up to the boot menu and ensure I was booting up on Zorin on 5.15.
Upon reaching the GDM Login screen, then ensure you are logging in on Zorin on X, not on Wayland.