I used Kate a note taking app synced with syncthing for a while. Using only md files. I had another md based app on Android that worked similarly.

Kate has a decent file browser for hierarchy and it'll stay in place and not return to a weird default path when you close it. And as you said, very fast to open and use.

For one off Notepad like things I like Mousepad especially because it has the Notepad++ feature of being able to save a session without asking you whether it should. Featherpad is also nice for this kind of use.