Factorio

I recnetly started playing a game called "StarSector"

Basically Mount & Blade in space someone said.

Really good so far, have not played more than 30h due to lack of time to play due changed life/work situation.

Edit: forgot to say that it is Single player but tons of mods that feel very much in line with the game.

Endless Sky is a pretty cool open source space game.

Also Dwarf Fortress.

Openttd trains for ever

Factorio is great, how has no one mentioned it yet? Also Dwarf Fortress, Into the breach and "FTL - Faster Than Light".

They all have native ports and are moddable.

Factorial and FTL even have huge overhaul mods, that change most stuff so they don't really get boring.

Kenshi. Sandbox rpg, citybuilder and more. Pretty unique and unforgiving, but the replayability is endless if you are into that sort of thing (lots and lots of mods as well).

I've played Stellaris for years, although I am really not liking the direction Paradox is going with their AI, and it really puts me off of their games.

Minecraft: Java Edition. You need internet the first time, but afterwards it's fully playable offline and also has LAN support built in.

Vintage Story. It's a very good survival, very customizable experience too.

I think it's a game that shines best in a multiplayer civilization RP tbh but even as a single player game it's a very nice survival. It may look similar to Minecraft at first but it is very very different and in all the good ways.

Plus mod support is so good, by design, even the website itself has 1-click installs.

Oh and it has chiseling.

Satisfactory, building game, a bit heavier on resources since it's 3d but it's good

Starbound! That game kicks ass.

Rimworld.

NetHack

Endless fun

Valheim has a local server you can set up that has it work offline. Shame it's not on gog, but it's on Humble which I don't believe has online checks like Steam, but I had it working with steam offline mode.

There is Luanti if you want something Minecraft like. It has a built in modding API and tons of mods available. It supports multiplayer over the LAN. It's also very lightweight, it will run on almost anything.

Do you know of Songs of Syx?

A bit like a large-scale rimworld, with a focus on managing a big population and world diplomacy/war.

A bit hard to get into, though.

Also, what about project zomboid? A bit far from your description I guess, but who knows.

Starbound and Terraria come to mind. Also Planet Centauri and maybe Stardew Valley.

Also Minecraft if a DRM-free version other than the Pi Edition (which is abandoned) ever comes.

Caves of Qud. I's like if Dwarf Fortress' adventure mode was actually good, and set in hyper-post-apocalypse scifi world where the very water you need to drink (or if you're amphibian, splash on your skin) is also the world's currency

Xcom 2: War of the chosen

Maybe CDDA with some difficulty setting changes to reduce combat difficulties and push it more on survival/building.

The Planet Crafter seems pretty neat, though I've only played the demo. It's a first person survival crafting game where you terraform a planet.

Technically, Satisfactory (3D Factorio, but more chill) allows you to host your own servers. You can host a server in your LAN, connect to server via IP -> enter the server's local IP.

There is also Terraria which is very low spec.

ICARUS. Works perfectly under proton.