OK, you are going to haul stuff, and you are going to trail ride. You are not going to DH at speed with your camping gear strapped on ... So you need comfort and efficiency. 15 PSI for DH is not in that envelope.

I'd say 29er for the ease of roll-over on rough stuff. Id say minimum weight because you'll be adding weight with your touring gear.

I'd say 2x9 drive train because touring will put you into all sorts of terrain from flat pavement for miles to roots and short climbs while laden. You need gears.

I'd say air suspension up front and Thudbuster out back with a rigid rear triangle for weight savings and rigidity while carrying you and the load. Skeleton rack out back well bolted with SS fasteners to carry touring stuff.

Carry a shock pump and adjust the forks and tires as needed. Air up for the road so you have good pedal efficiency and click in the lock-out. Drop a bit of air for the rough stuff and open the lock-out. Run different air pressures for general trail riding vs. touring while loaded.

Same for tires. Need compromise. Schwalbe Marathon maybe in as big a size as will fit the frame. Big white reflector sidewalls so traffic can see you at dawn and dusk. Not great going DH, but prolly doable going slow. Fine on paved stretches at 60 psi. OK off-road at 40 psi and down to 30 in a pinch will cover a lot of trail surfaces ...

Really nice saddle and good bars that fit for hours in the saddle. Have fun