I understand core banking is (was) going towards APIs for JSON output due to regulatory reasons (PSD2 - to clarify, PSD2 doesn't specify JSON, just, it seemed t

I understand core banking is (was) going towards APIs for JSON output due to regulatory reasons (PSD2 - to clarify, PSD2 doesn't specify JSON, just, it seemed the easiest option where I was) and just letting the rest of the code base sit there / be maintained. Layer upon layer, not great but who wants to re-write a general ledger. Consistent with your comment. OP might be in a sweet spot with this given JS experience and a switch to finance.