I'm gonna drop the Craigslist recommendation if you have a decent population of riders where you live. I bought by bike two years old for almost 1/3 of new.

You will need to know whether you want a full suspension or hard tail. The interwebs can tell you whether the component set is desirable or not. You've spent a ton of time in the saddle so you probably have an idea what comfort is on a bike (I guess? I don't know if MTB feel weird to roadies usually or not).

I will say the air fork and decent hydraulic brakes changed my world for the better. Where I live it's basically a smorgasbord of 29" bikes - HT and FS. Fat bikes and other tire size aren't plentiful on the used market yet where I am. Results may vary.

May wanna check out Bicycle Blue Book if you head down that road. Just my $0.02.