Dark arts mastery: Created a Linux x86 VM on my M1 Pro MacBook Pro
Within that Linux VM, created a docker container of Ubuntu with some personalised stuff.
On that container, built another one with the OpenWRT builder for RPi 4
Used that docker container to build a new OpenWRT image
Booted it on a spare RPi 4 and restored the backup of my OpenWRT config to it.
Took my actual OpenWRT router down, inserted the flash card I'd just created, and powered it back up.
Everything. Worked.
If you roll your own router, it's useful to know that you can recreate it were it to go tits-up, and I can!