To spell out the guys question, there is a silent "given I like the; DIY, elegant and utter simplicity of friction, ..." preface to it, or he wouldn't be attracted by friction drive.
Perhaps a typical context to empathise with - "I am a kid with no money, shop skills, car or licence from the flat rural (range anxiety with ebikes) part of USA or Oz (need to be 17 for a license) and i would like to spare mom the chore of driving me and the freedom of wheels. What I do have is a motor and a MTB. What should I do?"
Its a no brainer even if mom has to collect him in the pickup at times.
Chain is one solution to slippage, but it introduces many problems/complexities/hazards of its own. Clutch especially. Most need a means of disengaging the running motor from the drive mechanism. Chain guards?.
My rig was so simple yet practical once i didn't have to fret about rain. I had the best of both worlds.
The alleged downside of tire wear is a furfy on balance. Its a five minute job replacing a very "meaty" knobby $5 rear 26" MTB tire maybe twice a year on a qiklock wheel. The tire serves triple duty as also, a clutch plate, starter and a torque converter (at times where some slippage is in fact, desirable).
Soo simple. A brush cutter 2 stroke motor (a boon is they dont have float carbies and can be upended with impunity) and fuel tank on a swing arm from say, the seat post. The spindle is simply raised or lowered on the tire..
To engage/lower the swing arm, I had quality rope connected to a rearmost point, then down and around the lower rear fork arm, up to a quality cleated single wheel pulley (as used on kids trainer sailboats ) thru the pulley attached to the steering stem, and thence tied off on the seat post.
So you now have rope running parallel with the crossbar from steering stem pulley to the seatpost. Engaging the spindle with force is easy as you ride, simply use your body weight to push down on the crossbar rope length, and the pulley cleats lock it there.
To disengage the spindle, i used the very adjustable and versatile advantages of links of a length of chain and a small (3-4") mini trampoline spring (throwouts) from the rear of the swing arm to e.g. the seat, so when the downpull rope was released, the chain/spring raised the swing arm sufficient that it doesn't slam down on wheel on major bumps while raised. A little bumping is tolerable, the tire cushions the force.
So simply free the downpull rope from the grip of the cleats with an upward tug on the now slack crossbar rope, and the swing arm rises and disengages the motors spindle.
The swingarm was simply a piece of steel plate (could be improved on weightwise), cut with oxy to shape and drilled for securing it to only one side of the engine housing. No complicated second bearing on other side of the spindle, nor was it needed (it lasted forever til stolen). The spindle simply stuck out at right angles from the motor.