Been messing around with the Steam Deck with a prototype Unreal Engine project that I'm working on (and have a Steam App ID for).
Been needing to get around to implementing native SteamInput support (with full action based rebinding), but due to the problems that I've been facing with the amount of action origins there are (there's over 256 entries, and that's the maximum that Unreal Engine supports for enumerators in Blueprints), it's causing some unintended bugs with PlayStation controllers and the Deck. I might have to compromise and simply use it to detect the controller type to display the correct prompts, as sad as that sounds, but I'm hoping either Epic gets around to updating their Steam Controller plugin to be up to snuff with all of the improvements to SteamInput since then, or Valve gets around to making engine plugins for Unity/Unreal/Godot for that kind of stuff.
That said, based on my time trying to hit 60FPS, a few things I'd suggest doing:
"sg.EffectsQuality" has a notable performance impact, and I'd suggest lowering that as much as you can without coming at the cost of image quality. Should be able to adjust that in the GameUserSettings.ini file used by the game, or forcefully by adding it to an Engine.ini file that's packed up in a .PAK file in a "~mods" folder that would be in the game's "Content/Paks" directory.
I'd suggest enabling temporal upscaling and using a 75% resolution scale for a 540p output while not affecting the UI resolution. You can probably do this in the Engine.ini file while putting "r.TemporalAA.Upsampling=1" under a "[SystemSettings]" field. Some games don't support DLSS or FSR (Idolm@ster: Starlit Season comes to my mind, as that has raytraced shadows and reflections as options but no ability to go below a 100% resolution scale through the game), so config tweaks are going to be the best you are gonna get. I ended up using this with the Anacrusis back when it was in beta to get close to 144FPS on a GTX 1060 that I was using at the time (on a 1440p display), and it definitely softens the blow of the filtering done with upscaling.
I'd also suggest disabling SSAO and (if you don't mind) screen space reflections, as those don't particularly look great at low resolutions.
With those tweaks, I have no issues getting 60FPS in Scarlet Nexus, Tales of Arise, Dragon Quest XI S (keep in mind, this is with some of the mesh restoration mods), and other Unreal Engine games.
If you want to quickly move files over to the Deck, I've been using the SteamOS Devkit Client alongside Filezilla to FTP files to the system, so I don't have to boot into the KDE desktop mode to do config changes. Probably easier than using something like Warpinator.