Thank you for the input, especially the cancellation of CC payments (" Stop payments "). Apparently it is now 15-35$ depending on the institute.

Unfortunately from what i could tell the recurring customers arent really the problem, so they likely wont need a downpayment until they missbehave. Its more a question of how to deal with first timers who never become customers but eat up time slots. The down payments also wont make up losses in revenue (they would have to be too high) but would be more aimed at reducing the number of no-shows. So it would have to be aimed at those most incentivized to back out.

edit: The more i think about it, the more i am just describing an over complicated upfront down payment for first time customers. Thanks for pushing me to that conclusion.