You can use a mini bluetooth game controller as an iPad hotkey gadget

Check out this video of artist TerraTerrific using the miniature 8bitdo Zero game controller as a bluetooth hotkey gadget on the iPad, executing commands like "undo" and switching tools without having to interrupt the painting hand.

I bought one of those 8bitdo zero controllers to use for clip studio shortcuts on the iPad and uh...I dig it. pic.twitter.com/mBhjUjJq9g

- That's a thing y'know and it's gotta hurt. (@TerraTerrific) April 16, 2019

Unlike a standard bluetooth keyboard or full-size controller, the 8bitdo Zero fits easy in the palm. And unlike specialized art gadgets, it's cheap.

I reviewed it a while back. It's nice, but so small it's difficult to play with, and a pain to pair. So this is a better use for it!