I dislike pre-rooted builds myself. When I say a rooted build, I am speaking of a build in which the TWRP ROM installer includes a script to install Magisk systemless root as soon as the ROM installation is complete. That way, boot image patching takes place at the time of installation and future Magisk updates won't be buggy or problematic. It's an option for the lazy guy more or less. I always like to make builds without GApps, without root, and without any modifications not already baked in. You and I are on the same page there. The rooted build with Magisk script installer included is nothing more than a lazy man's/woman's convenience, but never technically a pre-rooted build. As you pointed out, those are always problematic in some respect.I respect such ROM devs who respect Google's directive and concession (that originally saved CM) that proprietary GApps should be loaded separately by end user (with licensed device) and not distributed with any ROM.
I'm not sure if there's any benefit from pre-rooted builds. They often cause difficulty / issues updating Magisk / ROM (see Magisk Discussion thread) especially when unpatched boot image is not available, and because John didn't design Magisk to be part of system.
It'll be very happy to see / interested in any development you produce! Many thanks in advance. PW