OP said their son in his friends use Xbox Series X/S, so that effectively removes Krunker and Minecraft Java Edition. That said, I believe Minecraft Bedrock Edition is available for XSX.
Stardew Valley is great, but the son’s social group has 5 people. Stardew Valley only supports up to 4.
Cities Skylines does not have multiplayer.
Can’t speak for Dorfromantik, because I haven’t played it, but the rest seem like great choices.