Using the latest version Proton 7.0-6 and SteamOS
After testing running with different settings. These are the best settings you can get at 40 FPS stable the majority of the time.

TDP: 8w
GPU Clock: 1000mhz

From top to bottom.
FSR 2: Performance
Texture Quality: Medium (0.5GB)
Texture Filtering: Medium (Trilinear)
Mesh Quality: Low
Shadow Quality: Low
Shadow Cache: on
Contact Shadow: on
Ambient Occlusion: FidelityFX CACAO
Volumetric Lighting: Mid
Particle Lighting Quality: High
Bloom: on (Personal Preference)
Screen Space Reflections: off
Subsurface Scattering: on
Hair Strands: off
Graphic Dismemberment: on
Persistent Corpses: low
Corpse Physics: Low
Diverse Enemy Animation: on
Motion Blur: off (Personal Preference)
Rain Quality: Low
Terrain: Off
Destructible Environment: Off
Lens Flair: off (Personal Preference)
Lens Distortion: off (Personal Preference)
DoF: off  (Personal Preference)
Resource-Intense Lighting Quality: Low
Resource-Intense Effects Quality: Low 

Frame Rate: 60/Variable (use the Deck's framerate limiter instead)
Display Mode: Full Screen
Cinematic Resolution: 4K
FidelityFX Super Resolution 2: Quality
Texture Quality (Recommended VRAM): High (8 GB)
Texture Filtering: High (ANISO x16)
Mesh Quality: Mid
Shadow Quality: Low
Shadow Cache: On
Contact Shadows: On
Ambient Occlusion: FidelityFX CACAO
Volumetric Lighting: Mid
Particle Lighting Quality: High
Bloom: On
Screen Space Reflections: Off
Subsurface Scattering: On
Hair Strands: Off
Graphic Dismemberment: On
Persistent Corpses: Many
Corpse Physics: High
Diverse Enemy Animations: On
Motion Blur: On
Rain Quality: High
Terrain: On
Destructible Environment: On
Lens Flare: On
Lens Distortion: Off (chromatic aberration reduces image quality)
Depth of Field: On
Resource-Intense Lighting Quality: High
Resource-Intense Effects Quality: High