Actually, Excalidraw looks really neat!
It doesn't support advanced stuff like Mentions and has only very limited text formatting options, but if it's just about brainstorming some ideas and putting a couple of post-its on a whiteboard, one can get quite far with it. Plus: doesn't need a lengthy introduction as it is super intuitive.
Stack: React, Rough.js, TypeScript, Vercel
Would love to see this in Cloudron.
Edit: Already suggested in the Wishlist.