As one of the Thunder devs, I can say there are markdown libraries. Thunder is written in Dart/Flutter and there is a great library that we use.
https://pub.dev/packages/flutter_markdown
That said, and as others have mentioned, markdown is not as well standardized and it seems like just about every site renders it differently, so there are a lot of edge cases to handle. Lemmy also has several unique implementations of things, such as spoilers, superscript/subscript, and the ability to tag users/communities without a hyperlink.
In fact, one of the things Thunder failed on (table alignment) is a known bug in the markdown library we use. :-)
Thunder allows cross-posting! It should follow the web UI implementation (where the body of the new post has a link to the original, plus the original contents in a quote block).