TBH, you don't need all the gizmos & gadgets. I've done all my own work over the years, engine out & rebuild, gearbox rebuild, complete new brake system etc.
My 'garage' has been a roofrack with scaffold poles attached to allow a large tarp over & the whole car has been up on some meaty home made ramps, lifting the car 24".
I've only lent my tools out to close friends, certainly wouldn't trust them to just anyone.
I think the biggest problem with your suggestion will be insurance. Second biggest problem will be wandering tools.
Problem I'm facing now I've bought a new car is that I have to have a garage service to stamp the book for the warranty. Once out of warranty, I'll need the garage to tell the car its had oil etc.
Might be better if you had an area with inspection pits & people brought their own tools?
The 'on-hand' mechanic could be there to provide welding services (at his/her usual rates) whilst working on cars he/she books in?