I've not taken a tour.
Overall, Italy has become more casual - even for the locals. I saw many women wear slacks, jeans (mostly teens or 20's) or leggings paired with a pretty top or sweater, jewelry, nice shoes or boots. The Italian women have more of a fitted look.
Here is a suggestion -
How about an "ankle pant?" or similar (like "slim leg pants") - If you know what they are, they can be dressed up or down in your situation.
Black, brown and navy are usually the good basic colors. Also, "slim pants" can be made to be casual or dressy.
Since you are going in August, perhaps you have or can find a khaki color. Paired with a light colored blouse or some other top, you'll look a bit 'dressier' and feel cooler at the same time. Wear a cute summer shoe or sandals if you wear them, and you're good to go.
I love scarves and bracelets and a couple of rings; they are my go-to accessories. (I always wear a crucifix, so I don't usually wear another necklace.) A Pashmina is nice too for the cooler evenings. It will make a cute souvenir.
Keep in mind that in Italian churches, esp The Vatican and St. Peter's, shoulders and knees must be covered - Women and Men - in case you are wearing shorts - even "walking shorts." Capri's would be okay since knees would be covered.
When I went to Italy, I like to "dress up" a bit for dinner. I would return early evening to my hotel, shower or freshen up and change clothes.
- Adding: I would say no to the jeans - too heavy and hot in the summer. Unless you have the lightweight slim or skinny jeans; maybe wear those on the plane to and from.