I’m typing this to a Rebased server. Rebased is a Pleroma fork. It supports Markdown for entries. And my client, Mammudeck, allows Markdown input, when the server supports it. Why Gargron has not added Markdown to Mastodon, I don’t know.
Pleroma servers also default to a 5,000 character post limit. Why Gargron doesn’t do that, I don’t know.
Soapbox, the default web client for Rebased, has a WYSIWYG editor for Markdown. Mammudeck shows the formatted text in another window, as you type the Markdown.