ChatGPT came up with this to make the change persist.
# 1) Create a minimal plugin dir with just "generic"
mkdir -p /mnt/state/root-overlay/usr/lib/ModemManager/generic-only
cp /usr/lib/ModemManager/libmm-plugin-generic.so \
/mnt/state/root-overlay/usr/lib/ModemManager/generic-only/
# 2) Add a systemd drop-in to change ExecStart
mkdir -p /mnt/state/root-overlay/etc/systemd/system/ModemManager.service.d