It's probably best explained in a video on what may be the best youtube channel on OpenWrt-related things: https://www.youtube.com/watch?v=FFTPA6GkJjg
Or, if you prefer textual information: https://openwrt.org/docs/guide-user/installation/attended.sy... (OpenWrt's infra is kinda slow today for me, must be many people checking out the new release :))
Just use image-builder. Or better, create a script which uses image-builder for you.
For me, upgrading to newer releases for my 7 OpenWRT devices, is mostly just updating a version number and waiting for the build to complete (a few minutes at tops).
It's not really hard.
I may have gone in a bit over-the-top, with makefile and dependencies at all[1], but at the core of things, it's not really hard.