Thanks, guys, for all the great suggestions and support!
Now that things are roughly stable, I typically save off my router configuration after major modification. I don't take Netgear router updates anymore. (Netgear Tech Support actually followed my suggestion and now make router updates optional after they bricked my Orbi - and probably many others - due to their forced firmware update policy)
I will follow the suggestion to set the router address range to a /24 subnet, and I suspect a number of devices are going to end up unhappy for a few days. I have run into subnet issues in the past with my current /16 subnet and got things resolved, so I expect I'll face a few temporary issues when I go in the other direction.
If most of your devices use DHCP and are npt set locally on the device then they will pull the updated subnet the next time they renew from the Router. Worst case scenerio would be you just need to reboot such a device. This is why i generally set everything in my router vs local config.