Couple of quick things.

Can you even use venmo to send money to people to write checks on behalf of third parties? I thought there was lots of restrictions on that type of activity in general and limits in these person to person platforms in particular.

Are these folks setup as approved businesses with venmo / paypal / stripe etc? I've had good luck moving large volumes if you are a legit large business not violating TOS. Note that accepting money to write checks for third parties is almost always a violation.