Thanks! I installed TVHeadend Player from the Discover store in Desktop mode on my Steam Deck - very easy, worked great.
First time setting up I forgot to add the default port for tvheadend. I was surprised I had to add that to the URL myself by adding :9981 at the end, as well as having to type http:// at the front of the URL. Would a pair of inputs for [Server address] [Server port] be easier?
It's easy to use as a desktop app. I found that I could control some stuff with the keyboard (context menu key, arrows, page-up/down, F, Ctrl-Q), but needed the mouse trackpads for interacting with the player screen, and needed to exit full screen to change anything.
No "pause live" option?
Because I couldn't see an easy way to map controller buttons for all of the functions I'll stick to Kodi + TVHeadend plug-in for the game mode, although it's a bit more confusing to set up (if you don't know how Kodi works already).