i am experimenting with this at the moment. first of many tyres do not measure up as labelled, i.e.. some 35mm tyres are actually only 33ish, some 38mm tyres are 40mm and vice versa
i triedWTB nano tyres 40mm ( v good reviews) they don’t fit !
also tried specialised tracer 38mm – it just fits with a couple mm to spare
i have fitted specialised tracer 35mm ( which measure up as 33-34 mm ) these are a cx tyre but roll really well on the road. i used them on an adventure race at the weekend, fast on the tarmac and coped well on forest tracks, mountain paths etc. really pleased with them
BTW i bought a cheap carbon fork, 27.5 sized – keeps the geometry more consistent and allows a bigger tyre in front if required – it will definitely take the 40mm tyre.