+1 for Excalidraw, especially regarding the "multiplayer" aspect.

In general, if sketching it out, whether alone or with multiple people, using something simple I can click, drag & label is more than enough and helps me keep the flow. (I would stick to actually drawing - either on a whiteboard or something digital - but my handwriting is absolutely atrocious)

As soon as I'm done sketching and move over to actually documenting a concrete solution, something code-based like Mermaid - as you said - is much nicer. But both are entirely different processes for me.

In Mermaid, while I like it a lot, you lose control of layout positioning. Does the auto-layout position large diagrams (many shapes + connectors) appropriately?

The Excalidraw export format is JSON, so you might use that in source control just as well. Or include the format in PNG or SVG export (which draw.io supports also).

Excalidraw is very nice in it’s simplicity. A rectangle can do a lot there: a textbox, a window, a mobile phone.

draw.io (now called diagrams.net) has a sketch style that mimics xkcd / Excalidraw. You can try it here:

https://app.diagrams.net/

There is even a desktop app available (but the webapp works offline too):

https://www.drawio.com/blog/diagrams-offline