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In the beginning when I heard about 650 gravel thing I considered it in the same way as @miro_o, but later got concinced that wide 650b & narrow 700c knobby is the way to go.
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Is mud that big of an issue for you? Personally I find actual mud doesn't feature very often... CX tyres might be better on mud but actual gravel or dirt, rocks, roots, etc. you're better off on fat tyres. With aggressive knobs you can do mud too - just look at MTB. I have 700c/650b wheelsets too but view it more like road bike tyres and verging-on-MTB tyres, in which case you want fat off-road and narrow on road
If the stuff you're doing is more CX than gravel then I guess it makes sense to have 700c knobblies though
Big slicks are plenty fast enough but much more comfortable.
Skinny tyres do well cutting through mud.