You can re-sign it using https://github.com/chenxiaolong/avbroot

I don't currently have any root on the phone, but I reserve the right to add it or run the userdebug build at a later date

We could use it to install magisk, but that wouldn't make the build proper "userdebug" one.

I fully agree with your original comment - AOSP security model is NOT a proper solution to the security problem, and I'd add to it that it was also designed to be anticompetitive - Google can do what third party apps can't.

Android architecture is tainted by Google's business model and it shouldn't be used as an example of a secure operating system..