One of the key issues is weather cancellations, of course. How much
can you as an FBO pressure pilots without making the whole thing
unsafe.

Yes, that's one area in which I was hoping for some suggestions. You
certainly don't want to compel someone to fly in conditions outside
their ability. Possibly setting some weather minimums that will allow
cancellation without penalty. Crosswind, vis, ceiling, etc, depending
on the nature of the flight.

One real problem is VFR pilots that reserve an airplane for a week or
weekend, then cancel because of low ceilings, or other mild weather
that would not be a problem for a more experienced pilot.