So, here's a problem that likely no one has (except myself 😄).
First, some context: I use Venmo on both my phone and laptop. When on my phone, I generally want my Venmo transactions to have the Friends privacy level. However, when on my laptop, I usually use Private.
My roommates & I usually do large grocery/Costco runs. I make Venmo requests from my laptop; each containing a "receipt" of items bought — all generated using a very elaborate Google Sheet. I'll save that story for another time. Since my roommates likely don't want their list of items out there for everyone to see, I'll use Private for these types of transactions. Other than that, I'll generally use Friends.
Anyways, the goal is to have Venmo default to Private on laptop, and Friends on mobile. Venmo does offer a default privacy setting. But that's account-wide, obviously.