The Assegai is hands down the best gripping tyre I've used, however it does have significant drag. Sandy trails...? Not sure, If it's loose beach sand than any tyre would feel like a any other IMO... I always run a DD (downhill Casing) because anything less gets shredded on the first rock garden. I'd suggest going for the good ole DHF front and back. Generally abit cheaper than Assegai, strong grippy, rolls pretty good.
I never, never ride on the road.(bike lane) I use foot paths, median strips if transiting to trails. I'd sooner do a UCI downhill with no gear before riding on the road. It's suicide.
'Gravel bikes' (knobby wheeled road bike looking things) are a good bridge into MTB for roadies.
