The way I’m figuring it is like this. The 29+ bikes you are pretty much stuck with the plus tyres on them.
The Tarn has tyres on that aren’t ginormous, so if you wanted to put some thinner tyres on it for mud it probably wouldn’t mess the handling up too much and the lower BB height would be a plus for muddy areas.
However you’ve also got the option of going for a set of 29er wheels with mud tyres on and the bike would handle just the same and you’d have loads of mud clearance.
Round Huddersfield where I ride it’s mainly rocky. You get the odd bits of mud but not miles of it in long stretches. I’ve always run big tyres round here and yes it gets a bit sketchy in a bit of mud but you expect it and the trade off is worth it for the advantage in the rocky bits.
So the Tarn 20 should work fine as it is, and if I fancy a change the frame will accommodate it. Sounds good?