Pick up a late-model 26er second hand. The geometry will likely suit realrively flat/singletrack-y type terrain you seem to be referencing, and go to amazon and buy:
A narrow-wide chainring (colour of your choice)
A single speed converter kit (colour of your chouce)
A SS chain tensioner
Shorten the chain to suit nd ride it.
My 1997 Kilauea will soon be back to SS duties having spent the last 8ish years as a child-lugger. It’ll be nice to have an MTB i feel I can ride inteh winter agian, without having to worry about how much damage its doing to my chick/pivots on teh main bike (itself from 2006, so parts availability is likley lmited!)
EDIT: and if you don;t like it, you can always stick the cassette/rear shifter back on and run it 1x…