YNAB can sometimes feel expensive. And while I could use an OSS solution like Gnucash or ledger, I don't have the time to deal with the tech support needed to be able to share my setup with my spouse. Likewise, we extremely value the mobile experience. To me, YNAB is cheap and well worth the money.
You might want to try "Every Dollar" which is free and has a mobile experience. It's not open source, of course, but it might fit what you're looking for.