Actual Budget is a fantastic FOSS budgeting tool that you can self-host

Not affiliated in any way with Actual Budget, but I can't recommend it enough. It's the FOSS version of YNAB pretty much so if you're a fan of envelope budgeting it's a great tool. I'd even say it has quite a few other strengths compared to YNAB (free bank syncing in the EU with more banks supported for example), and you can always be sure that your financial data stays within your reach.