Simple as that. Currently dungeons are designed so that a single player or a duo can do them. And in the current balance, an experienced player will do it fairly easily, too, even solo.
But when a group of five enters a dungeon, it’s just a completely underwhelming experience. Even if they aren’t geared to the max.
I don’t know how easy it is to add difficulty scaling to the dungeons given that the dungeons are not separate instances, but it should be something considered nevertheless as it would greatly enhance the co-op experience.
You’re looking for a private server for something along these lines. With that said, I’ve actually designed dungeons with 4-6 players in mind and can honestly say CE isn’t really a game made for that. When you get past 2-3 players the others simply start getting in the way. In addition the difficulty has to be ramped up that the margin of error the players can have gets really slim.
But if you want to see some actual group content. Use the Savage Wilds map and look for the Leviathan Bosses and Alaric the Icy.
You can also try Age of Calamitous and Endgame Extended Weapon Arsenal if you want to stick to Exiled Lands. These mods do work on Siptah and Custom maps but require extensive knowledge of how they work in order to set them up on those maps. They just work on EL.