Many good hiking backpacks under 40L will fit within the airline size criteria. The official dimensions of the pack are usually based on a fully packed backpack. If you leave the top pocket empty, for example, a too-long length will become acceptable. Comfort is the most important thing, and you really need to carry it loaded for a few hours, a couple of days in a row, to know how it is for you.
Are you accustomed to carrying a back pack on a daily or frequent basis? If not, you should be especially careful in choosing one, and testing it with 6-8 kg inside. You said you are planning your "next Camino" - if you have already walked, what type of back pack did you use? Also, are you planning to carry the pack or have it transported along the Camino?
I do see that this pack has both a sternum strap and a hit strap, both of which are essential for all day walking, so that is good.