So, its not DeX, per-se, but, I have been using Samsung Flow to my PC. Its pretty easy to set up. Its literally just a screen capture to your desktop with click functionality = tap. It is a bit laggy, so I wouldn't suggest PK'ing or bossing, but for like non-danger skilling (i.e. wc at guild) it works just fine.
Just flying by to tell that atleast for me the OSRS mobile on DEX now supports mouse and keyboard. Samsung galaxy S10 connected to a HP laptop with external mouse and keyboard. Don't know if its common knowledge. Even the default *-keys works and right click.