I was able to do some Steam Deck testing this summer, and I'm happy to say that Meanwhile works great.
Left-stick to select a panel, A to advance. Menu button to bring up the menu. All smooth. Or you can play directly on the touchscreen by tapping panels. That works too.
Enjoy your Steam Decking!
Note: The previous release of Meanwhile used a launch dialog box that you had to click through. As of version 1.2, this dialog box is gone and the game is 100% controller-playable.