So it had been pretty usable just sticking on 8.13 forever.
The one modem would occasionally not load SIM cards in it, but restarting the router would solve the issue. It still takes longer to do resets than the other, but that’s livable.
We had some downtime where the router wasn’t mission critical, so I tried updating to 8.4.0 and then 8.4.1. So far there’s been zero issues, so whatever behavior was introduced in 8.2.x has been fixed.
long related thread Peplink loses internet connection intermittently wireless and wired lose connectivity - #43 by erutan