There's a ton of these right now. I did some research the other day and found at least five "open source" (to various degrees, all of these are not strictly speaking open source but open core) notion alternatives, and they're all in some ways better, in some ways worse than notion. I settled on AFFiNE [0] because it felt the snappiest, but they've got a lot of telemetry and tracking so I forked to remove that and use my telemetry-removed frontend as a PWA.
[0]: https://github.com/toeverything/AFFiNE