In between 2.5 and 3.0: Which scripts/rules language to use?

I think you and Russ are agreeing Bruce. Only disparity perhaps is

Which I think is being developed just as fast as the version itself. (hats off to rich & others) I think good docs are as quickly forthcoming as stable release.

Not to disagree… at this moment (11.2020) 2.5 best starting point

Negative. You learned with existing doco for v2 which at one time was heavily disparaged but open source devotees pitched in and rocked it!!! (history lesson warning)
Edit: And really just to add… IMHO Yannick’s interface is so dog gone intuitive… it barely needs any explanation