So, far Zorin OS 17 worked fine on my old Macbook Pro 11.4 with me and once the notification for 18 came, I've installed the upgrade...
After the reboot, I was able to login, but basically nothing worked because the root file system was read-only. The dmesg command showed lots of I/O errors, which look like hardware problems...
I've repaired the file system, rebooted... still the same issue.
I've rebooted the system again and selected the old MacOS partition... No issues whatsoever. I've downloaded the Zorin ISO image and put that on an SD card to boot from... From the Live image, I was able to access my root file system with no issues. I've done some testing by writing a file from /dev/random to fill up the disk... No problem at all.
The kernel on the Live image is 6.14, the kernel on the operating system is 6.17. Using instructions from here, I've booted into the old kernel (6.8) and everything works as expected again.
So, anyone bumping into this issue... downgrade the kernel!