Spend more money on the shifter, that's what does the real work.
The rear mech is just a 'dumb' sprung mechanism & more likely to receive damage than a shifter
X0 or X9 shifter with X9 or X7 rear mech
Get whichever your budget allows, the increase in performance as you go up the range diminishes quite quickly once you surpass X9 level
I have used X7 to X0 rear mechs, only once broken a X9 rear mech, which was replaced under warranty. All other mechs have worked flawlessly
I've only used X7 and X9 shifters on my bikes, but having tried X0 on mates bikes if I had the cash I would use X0 shifters, they do just feel that little bit nicer than X9