I tried the overlayroot project @ https://github.com/chesty/overlayroot.
I found it’s method of writing to the lower level via chroot too much of a PITA.
I ended up using instructions from https://www.domoticz.com/wiki/Setting_up_overlayFS_on_Raspberry_Pi.
It has overlayfs for /home and /var with everything else mounted readonly. A job at shutdown syncs /home and /var back to the lower filesystem. Some of the locations from the author’s git repo have moved. I did a git clone, and then used find to locate the missing files.
Next, I need to setup a periodic timer to sync; the system won’t be getting many orderly shutdowns once deployed.