One option I'd recommend for anyone working towards this is to use the SimpleFIN Bridge [0], which is basically an API wrapper around MX (a Plaid competitor) designed for personal use by the same people that make Budgeting with Buckets. Data security is definitely an issue, but I value having my transactions automatically imported more than I'm concerned about the risk of SimpleFIN being breached.

I've personally used SimpleFIN to provide automatic imports in my own personal, kind-of selfhostable personal finance tool [1].

[0] https://beta-bridge.simplefin.org/

[1] https://github.com/avirut/bursar