On the GL.iNet download center I can download ready built images for my router.
According to support it is also possible to build (and customize) those images on my own.
However I noticed only an official imagebuilder repository which seems outdated and discontinued.
Therefore: Can anyone please share instructions on how to build those images?
Not possible.
The only way is directly with OpenWrt, or with qualcomm and your router is supported openwifi if OpenWrt does not support it.
You will lose the propetairy features such as the gl ui, and vpn software.
As far as support go, for OpenWrt directly you can get help there, for wlan-ap on their Github those versions are too different and not called real OpenWrt but a offshoot.
So, I understand that building the official image from source is not possible, because the source is not publicly available.
Is there an easy way to modify an official image by adding/removing/reconfiguring packages?
There is not unfortunately.
There are ways to get a list of installed packages, and reinstalling that list but it also combines core packages and some have appended version names in their names like libubox which breaks things, you only want to keep it at the user installed ones.
What one can do but that will be a little overkill, is asking gl-inet for the vermagic, then you can use OpenWrt buildtools with this vermagic and specify a diffconfig as profile which automaticly compiles your list of packages, then upload all of them to /tmp which is connected to your ram in your router and is gone after reboot and then install them all with opkg install *.ipk.
Please read about the vermagic here.
In my own situation I decided to skip the full gl firmware and build on OpenWrt directly with baked in configuration and all the packages, you will lose the gl ui and such though since that is propetairy.
here is my own repo, in setup.sh you can see where it gets the baked configuration from in the form of uci-defaults, I can now build my advanced network lab based on this and when I reset I still have the most stablest config.