[lvncelot]

+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.