Updated to Sequoia 15.0.1. Reinstalled the fix, and I have the same situation again whereas if the TB4 to TB3 to FW400 adaptors are plugged directly into the second TB4 socket on the Mac Mini M2 Pro, a get a crash-restart loop. Remove it from there, and then plug it into the TB3 port of one of my Belkin TB docks, it boots and the Onyx mixer is visible and useable. I'm pretty certain that if I was to try and restart with that connected, there's a good chance it will crash again. It's like I need to remember to unplug it before attempting a restart. Well, a little inconvenient at worst, but at best, I'm still using my lovely Onyx. :-)

Thanks for the info @daneover. Unfortunately, my experience with it has not been so good. The system went back into a startup-crash-restart loop again after a sleep that involved powering down my Onyx mixer overnight. The thing is quite power hungry, so I like to turn it off when it's not being used. I've disconnected it for the time being. I'm thinking of applying a razed earth approach and rebuilding my system and account from scratch to wipe nearly 15 years of migrated junk from it and seeing if that helps, but I'm dreading that. It is promising, however, that the fix worked as well as it did when it did. Gives us all hope.

FWIW, I did the Sequoia 15.0 (24A335) update today on my 2023 Mac Mini M2 Pro. Reinstalled the patch and authorised the kext from the pop-up in Privacy & Security System Settings. Shut down, plugged in the FW adaptors and my Onyx into a port on the Mac Mini, and got stuck in a restart-crash loop. Took it out, restarted, then plugged it into a spare TB3 socket in my Belkin TB3 Express Dock while the system was running, and there it is! My Onyx lives to survive another round of Apple's updates. So far it's been up for about 4 hours and working great.

Works 100% with my Mackie Onyx 1640i. Wonderful! SIP needs to stay disabled for it to work, so there's a risk there, but one I'm happy to take.