I have one «beefy» server/NAS (now it is EPYC2 on SuperMicro platform becuse they are cheap used on AliExpress, before it was Intel E3 12xx-badsed systems, two ir three generation old, always bought used). It is not my router, but NAS for all my data, NFS and build server.

I'm using FreeBSD, and here is script for preparing such installations named NanoBSD. To be honest, it is nothing special — build system with provided config (to strip it down, for example full FreeBSD installation includes toolchain and it is waste of space on «embedded» systems), mount file as loop device, create FS, install system to this FS by standard system means, add needed packages.

I'm building system once, make several images (as each device needs its own set of packages, of course), login to each device via ssh and run simple sh script which detects current active partition (by simply looking at output of mount command — from whic root is mounted) and then «dd if=/net/images/$hostname.img of=/dev/da0p$otherpart bs=128k», set this updated partition as «bootonce» in bootmanager and reboots. Last startup script in boot sequence check availability of network and liveleness of sshd, and if these simple checks are Ok, set this partition as alwaysboot (it is all UEFI features, mostly). If something goes wrong — one power cycle and device will boot from previous partition.

I don't have enough devices to automate «login and call script» part :-)