I’d probably use Stripe first if I built another FinTech. But it’s important to know that it’s viable to build your own ACH integration too. It took about a month of my engineering time to wire this all up for my last startup. (Not including contract time from the business side).
As a backup plan I’d suggest talking to a bank like SVB to see if you can set that up. (It’s always nice to have a warm standby payment rail if your business depends on it, though it’s really annoying to set up fully.)
For context the SVB integration is sending a NACHA file over sftp, you could do this manually for a single batch using the Python ACH library in an afternoon.
Yes, now you need to hold account/routing info, and collect PII like SSN/TIN. This makes infosec more expensive. But not as bad an option as going out of business.
On the plus side you can turn around debits much faster, use same-day ACH, and pay a few cents per transaction.
(If you need help on scoping/implementing this I happen to be available for consulting right now.)