The app you're looking for is Obsidian. Notion abandoned that goal years ago. Notion is making money for being a project management / team wiki. They don't care personal note taking.
Lotus Notes was the original offline-first everything app, including cutting edge PKI and encryption. It worked over dial-up and needed only a handful of MBs of memory (before the Java rewrite at least). Has anything else really come close since?
After a few rounds of AI generating AI content from AI content, I'm sure it could eventually become slop...like the model collapse lol idk.
"AI models collapse when trained on recursively generated data" - https://www.nature.com/articles/s41586-024-07566-y