Unpopular advice but: turn on the menus / use Emacs as a GUI program.
The menus are good and help you with stuff you may want to find, or things you didn’t know you wanted.
Alternatively? Escape X (M-xin “the lingo”): a type-to-find for certain kinds of commands Emacs can do. Can also list the keyboard combinations.
You can upload your statements and bills into it sure, but for budgeting it will help you as much as a piece of paper will: you can set up a notebook or a template or upload a (in your case register) PDF but have a calculator handy because it’s not a spreadsheet.
Depending on how complex you get it still may work. I could probably move my personal bill tracking method into the Remarkable, but some people expect Mint, which this won’t be.
I was initially worried about storage space for you, but I’ve been paperless almost a decade now and my bills collection is 500MB. So ?? should be fine ??