Sweetshell, don't think you have to use a backpack. You only need to be able to manage your luggage on your own.
You will find that most people traveling in Europe, including those on RS tours, use rolling bags of some kind. If you cannot comfortably carry a backpack weighing about 20 pounds fully packed, go for something with wheels or pack much lighter.
A couple of years ago I got the RS Appenzell backpack. I tried my best, but I couldn't get the weight down to <15 pounds, and even that was too heavy. I do use this pack for short domestic trips where the weight is <10 pounds.
Ten years ago I got an RS convertible carry-on backpack and used it on 2 European trips for a total of 3 months. By 2012, I was 66 and I was done with carrying that much weight on my back. My husband still uses his, loads it way too heavy, whines and looks for a luggage cart.
With a good, lightweight (<6 pounds) roller bag of carry-on size, that has a U-shaped handle, you can put a cross-body tote with a sleeve over the handle and roll them together at the same time.
No matter what bag you use, you need to stay within the carry-on size requirements which are the exterior dimensions including wheels and handles. Depending on the airline, the carry-on size could be smaller and have a maximim weight.
Next trip I'll be using this 2-wheel Eagle Creek Load Warrior 20 International Carry-on. At about 4.5 pounds, it's the lightest rolling bag I found that I could afford, on sale if course. Scroll down the page to see a comparison of it to the similar 22x14x9 one.
I'd resisted buying a bag with a T-shaped handle because of the inability to slip a sleeved tote over the handle and have it be stable. However, this bag has an "Equipment Keeper" that can secure a jacket to the top and I can wear my cross-body bag. At 2.2 oz, this year's cross-body tote is very light indeed.
I'm more concerned about weight this year because I'll be flying on 3 different intra-European airlines with different carry-on weight limits and I'm targeting the most restrictive of those.
Packing light usually concentrates on the number of garments. I also obsess over the weight of the garments. It sounds silly, but I weigh everything, pack the lightest things I have for my limited but coordinated travel wardrobe and weigh the packed bag when it's ready to go. Too heavy? I Ieave something at home.