I'm looking for any kind of suggestion, I'm interested in learning new things
At the moment I was thinking of using Tauri and to write my UI using some frontend framework, but delegate most of the parsing to Tauri, not sure if that makes sense.
Maybe evcxr would give you a head start, it's a Rust-based Jupyter Kernel. Jupyter has a fairly mature ecosystem with widgets and it's easy to come up with renderers for different MIME types, so you might be able to lift some ideas or connect with other people working on similar tasks.
ProseMirror was written by Marijn Haverbeke, who also wrote (and rewrote!) the much used [CodeMirror] editor. So the design of his "editor toolkits" are based on years of experience developing and maintaining text editors. Marijn is very responsive on all the respective discussion boards.
Examples, including markdown: https://prosemirror.net/examples/
Depending on your design, CodeMirror may be the better choice, especially if you want "parsing." The core of CodeMirror is based on [Lezer], a custom parser designed for editors. ProseMirror is meant to be the rich text editor, but I found my TaskPaper clone was more suited for CodeMirror because it was more focused on plain text. I suppose it would be possible to combine Lezer with ProseMirror, but ProseMirror already has its own [document model].
[CodeMirror]: https://codemirror.net/
[Lezer]: https://lezer.codemirror.net/
[document model]: https://prosemirror.net/docs/guide/#doc
https://ckeditor.com/docs/ckeditor5/latest/features/media-em... Though for Twitter previews you might need to use services like Iframely but there is a ready-to-use integration guide: https://iframely.com/docs/ckeditor
As for Markdown, you can switch the default CKEditor 5 output from HTML to Markdown: https://ckeditor.com/docs/ckeditor5/latest/features/markdown... And Markdown input is provided by a customizable autoformatting feature. It's enabled by default in all CKEditor 5 builds.