My friends and I are always looking for a game on Steam that would keep them busy for a long time. It should be a game that is not turn-based but a game like Minecraft where you can also see "progress".
Infestation The NewZ
Survival mode is the best, otherwise Battle Royal.
It's free, it's addictive!
I recommend 100% survival
Anno
or
https://robertsspaceindustries.com/
Ark is a really cool game
Sea of Thieves, but you should play two or three, because otherwise it will be difficult to get ahead alone. But there's no split screen, which means that your girlfriend also needs her own PC.
Destiny 2
