Is this not open source enough for your purposes?

https://github.com/mermaid-js/mermaid https://github.com/mermaid-js/mermaid-live-editor https://github.com/mermaid-js/mermaid-cli

OP's product is not open source btw

I’ve switched to D2 from mermaid after they removed ELK from the base. Dagre just doesn’t work for me. I wish there were better options.

Well, it is. Mermaid.js

Just vibe code a user interface to it.