Like many users, I've been hoping to see native batch processing for complex repetitive procedures in Paint.Net for several years, and it is clear that is never forthcoming.
The solution, which would have been obvious to many of you, is to use an external macro processor.
Not sure why I haven't seen a discussion of it previously.
I just found one that I can praise highly: it's as well designed as Paint.Net!
https://www.macrorecorder.com/