Looking for rules: Exclude all in this folder except some specified subfolders/files

First, see if that works for you. If so, great (made at least some progress…).

Duplicati has no default source, but if you mean there is more source than has been discussed, thus you really want Source to start at myRootFolder, I suppose this would work to grab the rest (but please test it).

I found a comment from the main author which supports my thinking about folder as source and exclude:

You can read through the rest of that topic to see if you can make any sense out of it to fit your situation…