I tend to prefer "file centered" apps, there's some cool one you'd use with Syncthing?
My favourite is KeePassDX; I would love a notes app with a 'material you' UI.
Obsidian is not FOSS, I still use it because I find it to be the most complete note taking app. You can download it from the Aurora Store and disable the network permission, same on linux downloading it from Flathub and disable the network permission using Flatseal