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