I once spent a month shopping around for such a thing. My team made a big analysis spreadsheet and we went through the various API's with a fine tooth comb and presented our top three to leadership.
They subsequently went with one we had never heard of. Rumor had it that it was owned by somebody on the board.
That is pretty much the case for any legacy industry, not just banking. It's 30% engineering, 70% actually knowing someone who will let your engineers anywhere near the data source.
In some cases regulations like PSD2 give you a "shortcut", but even then it's a sham because in practice the APIs you're given are dysfunctional and you will have to spend significant efforts making up for the shortcomings of the API (in some cases it's insane things like blatantly violating both the specification and even common sense, such as settled bank transactions outright disappearing without a corresponding reversal transaction).
Talk to one of you investors. They tend to be money people and relationship people, both of which make it likely to have a connection to someone at the bank.
Or just ping biz dev people from the bank on LinkedIn until you find an in. Someone will be thrilled to get a warm inbound lead.
Sure: one was a head trader for a specific prop trading desk, and the other was an executive director.
The GTM: the end customers are hedge funds/institutional investors, which are pretty hard to approach directly. So, we developed a B2B2B: we sell to large brokerage firms, which resell to the end customers.
I did this from scratch. There's a few huge problems. Do you have a charter? Can you supply the logistics for the regulatory compliance overhead? Have you selected an appropriate location in the FI pipeline to integrate? Not all banks have equal access to this pipeline.
Where does one even start? The only way I see is to stumble across someone who works in bank? If bank, has to be beyond a bank teller.