OAuth 2 MTLS Client Credentials

OAuth 2 MTLS could come in for when you're trying to do something really secure like what you're saying is backing transactions. There are two flavors of client authentication: regular PKI and the self-sign certificate. The idea was to try to accommodate different deployments and reduce some of the ease, not with the browser issues and usability, but with deployment and management of a TLS and PKI infrastructure.