Interesting idea to save as tldr and then save in DevonThink. Unfortunately, opening the file is not possible.
What I was thinking of is to make use of the “whiteboard as a service” functionality TLdraw offers. Next to offering their web app, they actively encourage and support the whiteboard being fully integrated into other applications.
Well, there is simply no application on MacOS that can open TLDR files. As far as I know, they can only be uploaded and opened directly in the TLdraw web interface.
However, while Mermaid converts written text (based on its syntax) into a visual output, TLdraw behaves more like Mural or Miro aka a real whiteboard app. As a user, you interact directly with the shapes by dragging, dropping, resizing, connecting them etc.
Here’s a link to the web interface if you’d like to try it out. As mentioned, it is possible to access this interface via a plugin from within StandardNotes. This would be awesome to have in DevonThink as well.