Have you looked into using your Visa to buy MCs that you use on Plastiq?
https://www.flyertalk.com/forum/30373410-post387.html
Supposedly there's only a 1% fee on debit card payments (per old posts), but I'm not sure if it's current. If you buy, say, $500 MC gift cards with a $5.95 activation fee and you can get a debit card fee from Plastiq well under 2%, then you'd be under 3% net. It would depend on how low the current debit card fee at Plastiq is as to how low your net fee (with both Plastiq and the gift card activation fee) would go. Also, some MC gift cards
may have slightly lower activation fees; I just used the $5.95 for $500 example because that's what I've paid for gift cards I bought at my local Ralphs here in SoCal.