I've been using YNAB. I want to switch out because YNAB is too expensive. What I miss with open source alternatives is the mobile app. I don't like to link my bank accounts, so I enter expenses manually whenever I buy anything. The mobile app is critical for that to work.
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.