Based on the fact that a 2:1 singlespeed can be ridden (with some heaving and grimacing) up most hills that a geared bike can, then using a 9 or 10 speed should allow most riders to ride almost anywhere.
If you have a 9 speed rear setup, I wouldn’t bother changing it for a 10 speed.
I removed my big ring from my geared mtbs a long time ago and have never missed them. 34t is plenty for me and the extra ground clearance is useful.
I did about half of my riding on a singlespeed for a few years so I know that I am capable of stomping on the pedals.
Having rationalised the fleet and becoming less fit due to a lack of riding for a while, I found that I was lacking a ‘normal’ XC/off-road commuter hardtail. I did about half of my riding on a singlespeed for a few years so I know that I am capable of stomping on the pedals so I raided my parts bin and bought some bits to install a 9 speed transmission: 34t Chainring and a 34-11t cassette. I really like it.
No, It isn’t geared for road riding, but then it isn’t a road bike.
I’m currently using an old XT mech as a ‘chain device’, but the chain does sometimes jump off on very rocky trails.
I intend to make/use something narrower and simpler as a top guide and to fit a lower roller guide.
On my fairly heavy full-susser I have kept my granny ring as it allows me to ride up steep hills whilst sat down.