I have 28mm Conti ultragatorduraskins or whatever they’re called on mine with full SKS mudguards. A719 rims. They’ve coped with plenty of unmade towpaths and sustrans tracks, not to mention a fair few Peak District bridleways… All good so far. My preference when things get rough is for more pressure rather than less, so as to prevent rim damage and pinch flats. As there’s no tread to speak of there’s never going to be much grip in mud anyway, and they grip fine on the rocks round here no matter what the pressure.
It has to be said though that if clearance is top of your list of requirements then the equivalent Surly is probably the way to go (better cable routing too). The Kaffenback’s chainstays were a bit tight when I did a cross race with a pair of borrowed 32mm twisters, luckily it wasn’t a muddy one though.