I still don’t quite follow. Markdown is just plain text, there’s no flag indicating whether a note is intended to be markdown or not.
Going back to your idea of a toggle: suppose we actually add one. When it’s on, markdown gets rendered; when it’s off, the raw text is shown. Even then, when the toggle is on, the client still has to decide whether to treat a note as markdown, because it has no way of knowing the author’s intent.
So you end up with the same situation as now: the client needs to rely on detecting multiple markdown patterns to make a best guess.
If, instead, you render everything as markdown without any checks, the toggle would likely be unusable, too much normal text would be misinterpreted and displayed incorrectly, so most people probably wouldn’t turn it on at all.