The cage of the derailleur is not the closest to the ground on the biggest cog nor on the smallest, it is closest to the ground when the cage is vertical what happen somewhere in the middle of the cassette.

The cage of the RD-R8000SGS is slightly shorter than the cage of the original RD-T610-SGS derailleur mounted by R&M on the Birdy Touring.

No, a 12s derailleur won't work with a 11s cassette and especially not an electronic derailleur like the SRAM Etap AXS.

But SRAM has an excellent 10-36 cassette with very nicely spaced cogs, a 10t smallest cog that provides a good gear inch with the 52t front chainring and Big Apple tires. Actually, the main reason I was interested by the SRAM Etap AXS derailleur was the existence of this cassette and this cassette works only with SRAM 12s Etap AXS derailleur (its the problem of 12s: no compatibility between different manufacturers nor between road and MTB derailleur, cassette and chain from the same manufacturer).