This is absolutely where we want to end up within the next year, especially as we move upmarket. Enterprises love having redundancy between payment providers to use as leverage for lower fees and there are several companies founded purely around payment routing, (deciding which payment provider has the best acceptance rate based on region, issuing bank, MCC etc). Some examples are primer and payrails.

It just turns out that most of our users are only really interested in using Stripe at the early stage, so for now our codebase is closely embedded into their billing product.

It's going to be fun(?) ripping all of it out and moving towards a PSP agnostic architecture you mentioned.