Tubeless vs Tubeless ready
Proper tubeless (UST) tyres do indeed hold air well, but are also heavy and don't roll as well (due to extra material that needs to deform around objects).  Due to the nature of MTB'ing you are also likely to need sealant for puncture protection, as whilst UST tyres are more robust, they can't shrug off all potential punctures.

This is why you see very few UST tyres available for sale now, with most riders (and manufacturers) preferring tubeless ready.

In my experience, a well set up tubeless ready tyre with sealant holds air at least as well as an old tyre/tube combo.  I typically lose ~2psi/wk on my tubeless ready setups so usually top them up once a week, but could go 2 weeks if necessary.