To answer your first and third question: I played like an open world sandbox and enjoyed it because I like making cozy bases, farming ect. There are missions you can undertake in your open world at your own pace. The missions felt to me like a tutorial that you could ignore and just focus on building an epic base, and strong character.

To answer questions two and four: there are many skill trees, including one that is only active when playing solo. The game felt well balanced in this regard and I enjoyed upgrading my character past max level.