Steam Deck Optimized Settings
This section will be quite brief.
To get inconsistent 30 FPS performance, apply the settings shown above and set TSR to Low. You can bump up textures to High, but I don't know whether that will have negative impact to long-term stability. Personally, I suggest using low texture quality which still looks rather good on Steam Deck's tiny screen.
If you want to get a frame or two extra, just use the Low preset and TSR set to low in combination with 800p resolution. That said, even then, you won't see stable 30 FPS performance, with some areas having the frame rate dip to low and mid-20s and staying there.
You can reduce the resolution scaling to 75%, which destroys image quality and doesn't quite fix sub-30 FPS performance in some areas but makes the game slightly more playable.
Whichever settings combo you opt for, don't expect the battery to last longer than about an hour and a half on the LCD version or about two hours on the Steam Deck OLED since the game will use every bit of power available.
You can do what you want with this information, but according to this reviewer, Silent Hill 2 is unplayable on Steam Deck.