Is there a reason why the large sums can't either be split or paid upfront?
*It’s unlikely that you will ever receive rover payment at the beginning, before the service even begins, in case there’s a dispute from the client which rover needs to review, which is why rover doesn’t immediately release payment on ending service date, and waits approx. 2 days after that.
For clients you generate on your own, you can opt to handle all booking & payment details (not converting them to rover clients) in which case you set all parameters,which may include deposit payment up front.
One suggestion is to split rover bookings into 2 or possibly more sets of dates. Hypothetically: Instead of booking Jan.1-31, book it as Jan. 1-15, 16-31, or possibly weekly bookings. In doing so, you’d receive payment for the first part sooner.
I deal with Airbnb and they are able to do that. I actually get paid in full the day after the client arrives and if there are any disputes they are handled either between the guest and host ideally but if that's not possible then ABB gets involved. It's tough to ask a first time guest to do that.