posts would be markdown files

Seedit, which is a plebbit client actually parses posts as Markdown, try it on Seedit

images would only be allowed as links to an image hosting platform It’s already this way with Plebbit, we only allow text.

Having it be open source and every member with a fork (I don’t know if there’s a way to auto update forks) so we don’t risk losing everything if the host shuts down (I don’t use mastodon because apparently you can’t export posts)

On Plebbit all clients are open source with GPL V2, they can also be self hosted easily with a single click. Check out seedit repository Seedit