He does not have to go with R8000.
(Cant wait till shimano release R7000)

all you need is the cassette from R8000, if you want to run a pie pan you can use any of the 11 speed MTB cassette (36T and up) They will fit on a 10 speed rear hub. You can use any 9 speed MTB rear dearailleur with 11 speed STI shifter.

One Oh five 11s GS rear with a compact front crank should have no problem with 34 or 36T cassette.