My kid with the AdventX conversion just replaced the rear derailleur with a "SHIMANO Deore RD-M5100-SGS Rear Derailleur - 11-Speed, Long Cage, Black" for $50 (Amazon). He managed to strip a few bolts trying to do maintenance on it (and broke my T15). He says this derailleur (nominally 11 speed, but the same pull ratio as the Advent X) actually works better.

The AdventX derailleur lasted about 2 years. (He is very hard on the equipment and rides a lot.) He found the high cogs in the cassette (Mk 1) wore rather quickly. Apparently they have addressed this in an Mk 2 version that will be available shortly.