Free: https://eqonomize.github.io
Not free: https://moneydance.com
I've used both at various times. I've found getting into the nitty-gritty for me is too much friction for me and I don't keep up with such programs. I just use a spreadsheet to capture my static monthly expenses.
Your efforts would be better spent contributing to an open source solution like https://eqonomize.github.io/ (or any of the several others linked in this post) rather than starting another one from scratch. That way you won't have an ongoing burden of maintenance, and your contributions can help all the people who already use that program.