I have one bike with 10 speed (Shimano- Saint shifter, XT mech) and one with 11 (XT shifting and SRAM gucci cassette) , tbh they both work extremely well. I think the wider ratio of the 10-42 cassette is the only real benefit I feel from 11 speed over 10 day to day though- and it can be quite a big deal, it let me add higher gearing to the big bike.
(I don’t like the 11 speed Shimano shifter at all, it’s a little vague and it’s just plain badly built, the 2 levers collide together sometimes while shifting, I wish I’d gone full SRAM but never mind)
Considering the price of 12 speed GX right now (£225) I’d throw that in as an option too. Or possibly instead of 11 speed?