Which Android app for drawing system design diagrams

On Android I’ve never found anything that feels as smooth as Excalidraw. Most diagramming apps on mobile either try to be “real whiteboards” with too many features, or they’re vector tools that fight you when you just want to think fast. If you want pure diagramming with a pen, Lekh is the closest like you said. The other route I’ve seen people go is using something general purpose like Infinite Painter or Concepts, then keeping your own little visual language for boxes/arrows. It’s less efficient, but at least the drawing part feels good. Personally I still end up using Excalidraw in a browser on tablet with the stylus. It’s not as slick offline, but it’s the only thing that doesn’t collapse under its own UI.