Oh! In my case I just had to switch the slot.
I restored the two bytes in the vbmeta_system_a & _b.
Code:
fastboot set_active a
And I got it all back without a full restore.
The hint for this was the modemst.
Apparently, they don't A/B that, just switch the contents.
Nope, this stuff is just rewritten all the time anyway.
So, my half-baked comparator did tell me something useful.