SimpleFIN is not free but it's very cheap ($1.50/mo, cheaper annually) and has an easy to integrate API. I use it with Actual Budget after giving up on Firefly III because of data import issues and it supports every financial account I have. Annoyingly several of my upstream financial institutions force me to manually reauthenticate the connection every time I want to import data, but that's my banks being stupid and otherwise it works flawlessly.

Firefly III at one point was planning to support SimpleFIN as well but it seems to have dropped off the radar. https://github.com/firefly-iii/firefly-iii/issues/5396