No hourly fees for glider-usage, but any active member of the club
pays an all-inclusive price that covers ALL the winch launches and
ALL the flying time (at the moment 259 Euro/year).

That's significantly less than any "beginner to solo" package I have
ever seen. The total (fees plus the all-inclusive price) is also lower
than the annual fee at many clubs.
So, given that - after the fees - flying is essentially free, how do
you balance supply and demand? Time limits? Long queues on good days?

Bartek