I’ve been interested in picking one up in the sale, but between the controls and newer games having high CPU demands, it seems like a stretch to get a comfortable experience in handheld. Something with either built-in controller support, a slower pace or a good community layout would be perfect.
What style of RTS are you looking for? City Skylines and Transport Fever 2 show up under Strategy for Steam Deck verified games, but I personally wouldn’t consider them a traditional RTS (although they’re both very entertaining to me).
mentioned in another comment, but Halo Wars is fantastic on the Deck. also, Command and Conquer Remastered plays well with some controller configuration.
Halo Wars. It’s designed for gamepad and the campaign is pretty decent. Co op works well too.
Tooth and Tail works well on a controller. It’s a simplified RTS and may or may not tickle your fancy, but it will play well. See some competitive play at https://youtu.be/CMGaSlUOX-0
You could give wartales a look, great game that I’ve sunk a lot of hours into. To be fair though I play on a linux desktop not steam deal but close enough.