Obsidian is a closed-source (reasoning explained here) alternative to notion that is bring-your-own-sync-service. You need no account, and all the data stays on your device unless you choose to employ a sync service.
All the data stays in a folder on your computer, which you can choose to share via SyncThing - an open source P2P file synchronization application. I’m yet to explore further but it is possible to set up web relays, allowing you to sync remotely.
Thought it was relevant - I’m enjoying it so far. It has a rich plugin ecosystem, including a Kanban board.
Check it out, let me know what you think!