Good news this time, I tried to be more systematic in how I approached this to try and figure out what was broken. So from a working state;
- Added switched to opencore and booted to make sure it worked fine (it did)
- Rolled back to the updated Clover qcow2
- Updated to UnRaid 6.8.3, VM now booted fine
- Updated to Catalina 10.5.4, VM now booted fine
So I'm now on 6.8.3 & 10.15.4 without issue. Without going back and doing more testing the thing seems to be having the updated clover during the update process has fixed the issue. No idea why that would be that case *shrug*
Thanks a lot for your help, especially around the quick responses and generating the qcow files it saved me a lot of time. Can I tip you a coffee?