I’d run 1×9 on all my bikes in an ideal world. Keeping the chain on, though, means it’s often less hassle just to go with the front derailleur and run a double up front. DMR make a rinky-dink chain holder-on-er that is cheap and cheerful. Will fit frames that don’t have a swing arm in the way.
I have a 1×9 set-up on my gemini with an e13 chain device like Duane which is the perfect solution, but it’s expensive. Ran me nearly 100 quid for backplate and chain device IIRC.
For a big bike like the gemini, it’s worth paying attention to the rear cluster when running 1×9. I have a small DH style one on at the moment, think it goes up to 28, and it’s tough work on the hills. It wouldn’t be a real problem on a XC bike but you notice it on the bigger bikes. I’ve just bought a 32 cassette that should sort that issue out.