Have you seen Kumu? https://kumu.io

It doesn't support standard UML diagrams but between sketch mode [1] [2] and icons [3] you may be pleasantly surprised. I personally use it to map out Kumu's own internal application structure and flows.

Full disclosure: I am the lead developer and cofounder of Kumu.

[1]: https://www.youtube.com/watch?v=wX3kbCyOamQ (Gene Bellinger's intro to Kumu's sketch mode)

[2]: https://www.youtube.com/watch?v=AFOz67co0yA (Benjamin Mosior sketching wardley maps in Kumu)

[3]: https://docs.kumu.io/guides/icons.html (Kumu docs on Font Awesome support)