@elder_tinkerer not with firmware-selector, but the image builder is supposed to add/remove files, see this post in the mentioned thread.
For me its enough though to remove the non-vectoring firmware so i have enough free space in the rw part of the overlay on the flash. Note, its not ram, it survives a powerdown of the system. Its the JFFS2 (or UBIFS) filesystem thats mapped via OverlayFS together with the readonly SquashFS part.
RAM filesystems are e.g. /tmp.
So i copy the dsl firmware once after flashing openwrt and its done. For just one router, thats very easy.
Going to my first 24h without disconnect. Of course, thats no timerange for judging, but its a start.