The stuff past beetle door is basically near endgame content. You're not going to find the keys for a very long time.
Lost Valley is way more open-ended, you can absolutely find yourself running into encounters that will likely kick your ass if you try to fight them immediately. (For example, I don't think it's recommended to go into the other dungeon in the jungles immediately after completing the quest chain to unlock it, until you are level 5+ and thus have access to level 3 spells. My own multiplayer party attempted it at level 4 and had to rely on some seriously lucky criticals/cheese tactics to beat the bosses down there.)
It's a potentially shorter campaign since there's multiple factions and thus multiple endings. The writing of each one is a mixed bag, but the combat encounters are top-notch.
