I recommend buying a travel backpack with a hip belt. The hip belt is essential for transferring weight from your shoulders to your waist and makes carrying a load much easier.
The second thing to look for is an internal frame or structure which helps with the weight transfer to your hips. My backpack has two heavy aluminum stays which is on the extreme side. An Osprey uses a lightweight wire frame. Others may use a plastic frame. It is possible to get away from using any internal frame by packing and cinching up your contents tightly. Packing cubes may help in providing some internal structure. As long as the bag is not packed loosely or sagging, most of the weight should transfer to your hip belt.
One nice feature to look for is being able to stow away the shoulder straps and hip belt in case you check in the bag. You do not want those straps flopping around in the baggage hold.
I don't own one but I don't think you can go too wrong with buying one of Rick Steves convertible travel backpacks; they are currently on sale 20% off. I own a SOC Bugout Bag, which is a decent heavy duty carry-on travel backpack, but it is marketed as a military bag. In my opinion the gold standard travel backpack would be an Osprey Farpoint, but they are pricey.
I bought a cheap 40L travel backpack off of Amazon for my wife who was considering switching from her two-wheel roller to a backpack. However, we found that it was considerably smaller than the advertised 40L and held much less than her roller. I should have known better; the length x width x depth measurements were shorter than average. The top was tapered and the corners are all nicely rounded. It looked good but a lot of the volume is lost. If you want to maximize the volume, the pack should look boxy to maximize the volume using the limited carry-on dimensions.