Very timely. I literally ran a Claude prompt "compare and contrast Notion vs Obsidian" and flipped over to HN while it was thinking, and this comes up. Thanks HN!
You could try https://hyperclast.com/ (my project). Here's the comparison vs Notion, Obsidian etc https://hyperclast.com/vs/
I self host https://www.getoutline.com/ instead, they might not have the latest AI features but it has everything I could ask for from a Notion alternative.
I switched from Obsidian to Joplin years ago. Its completely FOSS and can sync with your private Nextcloud instance.
Consider Trilium if the collaboration stuff people use Notion for isn't important. It's open source, uses SQLite, and does automatic daily and weekly backups.
For a personal knowledge base? I would stay far away from anything proprietary for personal notes. I love logseq though I'm increasingly worried it's abandonware