Capitol One JUST obtained Discover, which could potentially be in our favor:
American Express can be pains in the ass, but they are definitely a potential alternative payment processor.
Other alternative processors are:
GoCardless https://gocardless.com/guides/posts/open-banking/
Square https://squareup.com/us/en
Dwolla https://www.dwolla.com/
Adyen https://www.adyen.com/
Heartland https://www.heartland.us/
CardX https://cardx.com/
Helcim https://www.helcim.com/
PaymentCloud paymentcloudinc.com
Tipalti https://tipalti.com/
Revolut https://www.revolut.com/en-US/
Remitly https://www.remitly.com/
Hyperwallet https://www.hyperwallet.com/
Skrill https://www.skrill.com/en-us/
Oceanpayment https://www.oceanpayment.com/
Dots https://dots.dev/
Zenpayment https://zenpayments.com/
This isn't an exhaustive list by any means, as there are many others.