Yeah, it's an American thing mostly, I think (it originates from Quicken). I know that Germany used to have another standard, HBCI (which is now FinTS?) and I'm not sure what other countries do.

OFX is a huge PITA to implement, I've tried it and realized I would want to get paid to put up with it :) I'll try the CSV import, if it works then there's no need to overcomplicate stuff.

The Demo looks really good by the way, and props for actually having one!

Perhaps you can just use https://github.com/aclindsa/ofxgo - it seems to be a pretty good Go package (just eyeballing it, haven't use it).