It’s weird how the markdown formatting bits (such as ### in a heading) appear in previews of posts. I don’t have time to research it right now, but I’m guessing that’s a known issue waiting to be fixed by someone.
Edit: Trying to add a screenshot to show what I’m talking about, but uploading images doesn’t seem to be working. I tried 2 clients and the mobile web.
Your point about the possibility of unevenly sized text and line breaks mucking up the whole layout makes sense. I wasn’t thinking about that. I think a better-than-nothing solution would include the stripping out of markdown formatting that alters the rendering in, image references (I’m new here, are images even allowed in post body content?), and converting the markdown URLs as you mentioned.
Another thing that could influence what people put in there would be some example text in maybe some helpful guidelines alongside the post form.
Looks like at least I’ve got a few more reasons to finally sit down and pick up a little Rust.