How to become a custom software vendor of major banks?

Start making friends. You don't land big deals by being great, you land them by being good enough and being friends with the stakeholders. A lot of the time you don't even have to be good at all - just be friends.

In this way, the best thing for friend-making is to attend an Ivy League and be not autistic.

Do you know anyone who has purchasing authority at a major bank? If yes, talk to them. If not, buy a subscription to linkedin sales navigator and start cold calling until you know someone who has purchasing authority.

Oh man, this is the last thing you want. I was on the other side of this and we treated our vendors horribly.

A lot of friends and maybe kickbacks. I guess previous bank IT experience helps.