The Penumbra games scared the shit out of me

Agreed on Flutter, very easy to get going with it.

Currently Endless Sky! Although I haven't gotten far, and playing without mods, I love the vibes

A survival game akin to rimworld, banished or project zomboid except food is insanely realistic. Crops take ages to grow, hunting a deer should be a massive victory that secures you for a while, you could become nutrient-deficient by only plowing down heads of cabbage, and so on...