I haven't used it, but the team (person?) that makes Buckets (https://www.budgetwithbuckets.com) makes SimpleFIN (https://www.simplefin.org), which seems like it exposes exactly what you want: simple transaction data from arbitrary banks.

Plaid offers transactions APIs (https://plaid.com/products/transactions/), but I guess to your point these APIs are geared towards fintech companies, not personal use.

EDIT: I don't know how well it works in general, though. You'd have to test it yourself.