I was hoping that more people would offer opinions and I don't consider our site to be high volume but volume is relative.

Your processor list is impressive.

If we were processing a significantly higher volume, I would consider a change.

Our current processing volume is right at the tipping point where the balance tips toward using the PayPal gateway instead of auth.net gateway.

Options available to me:
PayPay WPP => $30/month regardless of volume
PayPay Gateway only => $25/month regardless of volume
PayPal Advanced (possible future option) => $5 per month regardless of volume
Shift4 => $25/month plus volume
Auth.net => $15/month plus volume

Have you considered approaching the multitude of auth.net gateway resellers? If they had 2 competing gateways to offer customers, things could work to your advantage. I assume that many/most merchant providers also re-sell auth.net accounts at a discount. Merchants only realize there is a problem when they change processors and the original processor cancels their auth.net account. That's why we use gatewayonly.com so that we can change processors and not worry about our gateway account being cancelled.