Look into opening an account on an Iceshrimp instance like fedia.social, or an Akkoma instance like pleroma.envs.net. Their APIs are an extension on top of the Mastodon APIs (and both support Markdown and should expose them to apps). Sharkey also has some Mastodon API compatibility but it’s quite broken and might require some odd workarounds to get stuff working.
That said, for rich text, parsing the HTML will be more than enough for nearly all cases.