Nice work! I've been mulling something like this for awhile. I use YNAB (classic) to track our budget, but I haven't made the switch to their new subscription-based version because it just doesn't fit my workflow as well. The experience made me think more deeply about what I actually need in a tool like this, and I came to a similar conclusion that you seem to have: I need a convenient way to enter my expenses and balances, and a way to analyze them. A spreadsheet is perfect for the latter purpose, but isn't very good at all for the former.
Having said that, the thing I care about the most in YNAB is importing expenses from my bank accounts. My thought on replacing this was to be able to add download URLs to my spreadsheet and kick off a script which would prompt for manual username and password entry (so that the credentials aren't saved) before downloading and parsing the exports. I haven't gotten around to creating this yet - if you added it to your project, I would seriously consider using it!
I came to a smiliar conclusion and have been working on a solution for my particular set of bank accounts. So far it downloads transactions from citizens bank or venmo using selenium. Then in a separate stage it parses them into a python object based spreadsheet.
I've also used YNAB and loved how it could categorize spending. So a part of my project is that it lets you use udev-inspired matching rules to assign categories or other properties to transactions.
You can also put credentials for each bank in an encrypted file behind a master password. I can't promise it's secure but it's enough for me to not worry about saving the info to disk.
There's still work to do: I've only implemented a couple bank backends and some core logic, and haven't got around to adding anything for analysis, or a real setup.py, or a readme. Still working on it a bit every day. Fidelity is next. Would love it if anyone was interested and wanted to add their own equally jank bank backend.
Would switch to this immediately if there was a good UI for iOS that kept in sync via Dropbox or some other method. I was disappointed that YNAB ditched the old model and decided to go towards subscription. I am currently using YNAB (Classic) and am looking for a way out because I understand that it will eventually die. There doesn't seem to be a good replacement that has good iOS and Desktop applications (GUI or CLI).
Having said that, the thing I care about the most in YNAB is importing expenses from my bank accounts. My thought on replacing this was to be able to add download URLs to my spreadsheet and kick off a script which would prompt for manual username and password entry (so that the credentials aren't saved) before downloading and parsing the exports. I haven't gotten around to creating this yet - if you added it to your project, I would seriously consider using it!