Oddly I’ve just de-built a Soma Double Cross and put on the bits on a Dirty Disco frame, it built to a genuine 20lb without any fancy bits and running Hope Hoop Crest 29ers. I’d say it’s comfortable enough, takes discs obviously, I’m running a 34/50 compact but with an 11-32 mtb cassette on the back and it’s fine on and off road and quick.
It doesn’t have similar angles to a mountain bike – or at least a modern one – but it’s certainly not twitchy like a racy road bike.
Touring? I guess it depends how you define that. There are no rack mounts etc, but then again Mike Hall rode a DD round the world, so it clearly can handle some big miles.
I really like the thing and being able to ride on and off road on the same bike – I have a mountain bike also and a proper road bike, so I tend to use those for the specifics – and disc-braked crossers make brilliant winter road bikes ime. Great at night on pot-holed back lanes as well.
I don’t know about the mtb geometry thing. Maybe something like a Cotic X, which felt sluggish up front to me, might work. On sizing, the 54 DD works for me at 5’9.7″ or something like that, but it’ll depend on your proportions and preferences I guess.
Don’t know if that’s any help, but I’m loving the Dirty Disco right now. All matt black and no stickers. Only slight concern is that the tyre clearance is tight at the back with 34s on Crests, one of the brake hose mounts is disconcertingly close to the sidewall.