safety

In crowded situations, you can carry your backpack in front. I've done this and seen others do it. I wouldn't place valuables in the pockets of cargo pants. Thieves know to look there. The money belt, worn under your clothing, will work best for money, passport, credit card, etc. I carry a wallet also for daily expenses. The wallet goes in a fairly secure pocket. I usually go to Europe with a travel vest that has multiple pockets. Some of the pockets are secure enough that I feel safe leaving the money belt in my room and carrying the important stuff hidden in the pockets of my vest.

Just be aware of your surroundings and you should be fine. Remember, the hundreds of thousands of people who travel to those cities every year who do NOT have a problem don't post about how safe they were, or felt. You only hear from those who do, so it seems like a bigger problem than it is. I was in Spain most of May, and in all of the cities you listed, except Barcelona. I never felt unsafe. I carried a daypack every day.

I wouldn't worry about the backpack. Those types of theft that you describe are pretty rare. Spanish theives are particularly adept at picking pockets, so having things in cargo pants pockets is risky. Have a money belt or neck pouch type system that they can't see or get to. The other thing to keep in mind is that the overall risks are small. Just being aware of your surroundings when in crowded spots, or walking packed areas like Las Ramblas is about all that is needed.

Agree with all of the above suggestions. When my daughter travelled solo in Europe, because she was in a room alone, she always used her door wedge. Easy to pack, and will give you peace of mind.