I thought about doing something similar some time ago, because I never quite found the perfect note taking app for myself. There's a million ways how to do notes, and it feels like there's just as many different notes apps.

Eventually, I've settled with Obsidian because of its simplicity and extensibility. You can leave it with basic features and truly own your notes in a simple format (you can also put them into any cloud, as long as that cloud reaches your filesystem). It doesn't do everything just like I'd want to, but I've thought about just building another notes app that reads and writes to the same path your Obsidian notes are in, instead of trying to cover every possible editing feature like most big notes apps. Then I'd use different apps for different needs, with one place to store data.

Since you're focusing on privacy, have you considered using Obsidian? Is there anything particular you want to do differently?