FlashFix My games all work on the Steam Deck to various degrees. I haven't made any changes to them to work on the Steam Deck - they are all Windows games only.
Bullet Heaven 2 is a Starling game with controller support and works perfect on Steam Deck.
Epic Battle Fantasy 5 is a Flash display list game, and the performance is not perfect, but pretty decent. I've heard that some of the resolution options cause some issues on the Steam Deck (I used quite an old version of AIR and an external app to change screen resolution). The text in the game is slightly too small for comfort. But I think the biggest issue is a lack of controller support, which means you have to play with the trackpads or the touchscreen - some people like them, but I personally don't.
So in summary:
Performance on the Steam Deck is decent, even with the Flash display list.
(If it's been optimized for mobile, it will probably run very well.)
Windows games run just fine, I haven't seen any issues with Flash running on it.
Controller support helps a lot.
Use a mobile-friendly UI with big text.
Don't do anything weird with resolution options.