Summer faves on my Cannondale 26er MTB were Big Apple 2.35. Heavy but rolled fast. Went all over Exmoor on them on and offroad. Can’t see why 2.15s wouldn’t be similarly fun/fast.
Schwalbe Hurricanes similar, with better offroad cornering thanks to knobbly edges. Bit of a frankentyre. I liked them
Land-cruiser 2.0” were pretty successful also on 50/50 but again a little heavy.
Of all I enjoyed the Big Apples most, I like balloon tyres for gravel.
I have 1.10 (28c) Marathon Plus Original (Performance Line Greenguard) on the current tourer and on gravel they are ok (swift and quite springy, grippy, decent protection) but better on the road.
ie volume isn’t really up to long gravel stretches with potholes at speed. I am a seriously fat knacker atm tho. Thinking of going up to 35c (1.35) which I predict would be nigh on perfect for gnarly country lanes, towpaths, unadopted streets, commons and fire-roads which is my typical riding these days