I don’t think shutters will provide much of an R value, but they might block out some sun. Window insulation film is a thing that exists, and might be your best budget bet. Window treatments also work well, and vary in price.

Anna’s Archive is always the first place I check

Not OP, but I love yt-dlp.

Vibrant Deck is nice because it bumps up the colors a bit and makes it seem almost OLED, but I think the actual best is Proton DB Badges. This plugin tells you if a game is compatible (and how compatible) with the steam deck. Steam official playability is notoriously conservative, plus Proton DB can tell you what might not work in a game.