Take it from me, I'm really picky about having a game looking great and performing well. RDR2 look excellent on the deck and plays great with no obvious stutter or frame drops. Real eye opener to the capabilities of the steam deck. Following setting I found online work great for me:

Deck Settings:

• FPS Limiter - 30 FPS

• Refresh Rate - 60Hz.

• All other settings default.

In-game settings:

• Resolution - 1280x800
• Vsync - off
• Triple buffering - off
• Texture quality - ultra
• Anastropic filtering -x16
• Lighting quality - high
• Global illumination quality - high
• Shadow quality - low
• Far shadow quality - ultra
• SSAO - ultra
• Reflection quality - low
• Mirror quality - high
• Water quality - custom
• Volumetric quality - custom
• Particle quality - ultra
• Tessellation quality - ultra
• Taa - medium
• Fxaa - on
• Msaa - off
• Near Volumetric resolution - low
• Far volumetric resolution - high
• Volumetric lighting quality-high
• Unlocked volumetric reymarch res - off
• Particle lighting quality - ultra
• Soft shadows - ultra
• Grass shadows - low
• Long shadows - on
• FRSSAO - on
• Water reflection/refraction quality - low
• Water physics - lowest
• Resolution scale - off
• Txaa sharpening - over half
• Motion blur - off
• Reflection msaa - off
• Geometry level of detail - second highest
• Grass level detail - lowest
• Tree quality - ultra
• POMQ - ultra
• Decal quality - ultra
• Fur quality - high
• Tree tessellation - off