Just looking at if you could possibly make more by allowing groups to accept donations. I'll present my group as an example.
We are a homeschool support group in the midwest, currently using the free plan. That level works well, but calendar RSVP'ing might be nice to keep track of who's attending the playdate, picnic, fieldtrip or whatever. The Premium plan would be nice to add for the calendar feature, but it's a little bit of a stretch given our group's situation.
We have roughly 150 member families. To keep it simple, I'll say 100 are regular members who pay $25 a year that funds group activities for the kids/students, leaving 50 who pay nothing as their kids are all graduated, but they want to stay connected to their friends.
The 100 pay their fees via Paypal which matches the fee structure you present for Stripe. If instead, I use your system, the extra .50 per member (your 2% cut) brings in $50 that wasn't there before. Of course, if my homeschool group bought the Premium plan, you would have that fee too, but that isn't currently being paid.
So, a possible alternative structure is maybe adding features (calendar RSVP'ing in our case) with enough payment process fees?
Anyway, just sharing some ideas that are popping into my head as I'm looking at registration and planning for my group for the coming school year. Mark, feel free to contact me directly if my comments bring any other questions on your end.