So I could avoid needing a Wolftooth if I had a medium cage RD rather than my short cage (probably). I think Wolftooth or longer B-screw + SRAM cassette is cheapest option for me.
But it also looks like I could run the XTR 11-40 cassette on my current setup if I fitted a Wolftooth.
Ultegra med cage with SRAM 1130 11-36: https://www.pedalroom.com/bike/carbonda-gravel-bike-1x11-di2-30292#photo239239
No wolftooth, B screw is about 70% in so if there was a 38t it would also work.
I don't think so unless you go for an XT (or XTR Di2) rear mech. Maybe @Emyr can clarify.