I like both Lucidchart and draw.io but I suspect I’ve been a little bit naive in my thoughts about a dynamic image; i remember that Discourse downloads and localises the image. Turns out that getting the image to update post edit has an awkward workflow at best.
Looks like you would need some sort of callback that would magically rebake the post with a new image version. Or would i be better off thinking about some sort of IFRAME style embed that always looks at the remote server for the image?
I’ll take another look at the Embed draw.io diagram in post option you mention; at first glance it looked like gobbledygook to me 