Is it better to travel to northern Europe with a group or alone?

This depends on your personal preference. From the point of view of saving trouble, you can consider joining a group. After all, there are many people with tour guides, which is safer and more worry-free. Don't worry about yourself. Just follow the itinerary. However, from the perspective of tourism experience, it is recommended to travel freely. You can go to many places that tour groups can't go, and you can also eat many special foods. And you can freely allocate time and travel.