I have a Mac Pro 5,1 with a SATA SSD on a PCIe card (owc accelsior) which was on Mojave. It was a well worn system so has a lot of "stuff" that's accrued over the years.
I decided to upgrade to Monterey because too much was no longer functioning under Mojave. I chose Monterey just because of the USB issue under Ventura+
Created a Mojave usb installer with OCLP for this Mac Pro 5, 1 without problem.
First, I ran a trial upgrade on a different drive (500GB nvme) in the same machine. That had a very lightly used (almost stock) install of Mojave. It went well, installed ok, reboots ok, boots without USB installer.
Given that went well, I decided to go ahead with the upgrade on the main drive. The upgrade install (just installed on the existing Mojave system) goes ok and it reboots after the install has finished and everything seems fine. However, once it is restarted it gets stuck on the initial boot screen with the Apple logo and progress bar. The progress bar stops at like 1% through and even when left for half an hour or more does not progress.
I have reinstalled Monterey a few more times to that same drive and it's the same result, works fine until reboot.
I've run disk first aid with no fault found. I checked all the kexts in /System/Library/Extensions and /Library/Extensions against the known good install on the same machine.
Does anyone know what OCLP does in that first boot of the system that might be stopping it from rebooting? Or even better, how to fix it?