Mac partition won't boot since Ventura upgrade, even in safe mode.

I have a 2019 iMac. I have the hard drive partitioned with two different instances of OSX, one for photography/video work and the other for music. This is mainly because I prefer an up-to-date OS for my photography work, while having a slower update schedule with my music mac to avoid compatibility issues with my man plugins/vsts. I also like keeping the two things separate. It all worked perfectly for 4 years.

I recently updated the Music mac to Ventura to get the latest Logic Pro update. When I tried to boot back into my photo partition, also on Ventura, it got stuck in a Kernal Panic loop and failed to get past the apple logo. After some failed trouble shooting I reinstalled the OS and it sorted the issue. I then updated it to Sonoma.

Today, when trying to boot my music partition it also got stuck in a Kernal Panic loop. I tried:

resetting PRAM

Disk Utility First Aid

Resetting SMC

Hardware check (no problems found)

Unplugging all peripherals

None of this worked. I then tried to reinstall the OS as I'd done for the other partition but the option was greyed out and 'this operation couldn't be completed (com.apple.buildinfo.preflight.error error 21)

I then tried to boot in safe mode but it continually refuses to do so. The Photography partition boots up every time without issue.

Apple chat support couldn't help but they said they'd escalate it and someone would ring me "within an hour" but that didn't materialise.

If anyone has any ideas/suggestions in the meantime I'd be eager to hear them. My instinct is that it's a login item that's causing bother, but I don't know how to address or fix it without being able to boot it up.

I have a time-machine backup but would rather make that a last resort.