You're funny. If one person withdraw, it doesn't prove anything, if 50% of all BTC are withdraw at once, it proves you have at least 50% of it, if 80% withdraw, it proves at least 80%.

The only way to prove it that way is that 100% of all BTC of all customers are withdraw all at once. And this would need to be repeated at random interval to prove it really.

There must be a way to prove the reserve otherwise self-custody is the only viable solution.