It feels like you know where you'd like to go but either your cautious inner voice--or perhaps a well-meaning friend---is discouraging you. If that is the case, go all in and do Scandinavia.

On the hand, if you really are ambivalent you could spend 4 days in a sunnier warm place and 5 days in one country of Scandinavia. The 10th day would be travel between the two destinations. Truthfully, you would spend part of a day traveling between Sweden and Copenhagen, anyway.

I was little surprised that Warsaw was a hub for whatever airline you are flying. That's not something we see here often. Also I would suggest you visit the flight section of this board for guidance on your routing. There may be better options out there, but I am in no way the person to advise you.

As to #4, my own wishlist includes Madrid and Barcelona.