Isn’t it a client issue and not a nostr note issue?
Edit: you could add an option to strip markdown syntax on cross post? That way you can choose to keep or remove markdown formatting? I would hope that nostr clients would eventually be able to detect markdown and render it appropriately at some point
The spec explicitly mentions clients shouldn’t parse markdown content in kind 1:
1: text note: the content is set to the plaintext content of a note (anything the user wants to say). Content that must be parsed, such as Markdown and HTML, should not be used. Clients should also not parse content as those.
I guess again it feels like a client issue. Either detect and parse and render markdown, or also support other kinds that the spec allows to contain markdown 🤷♂️