@sandraessex I sent you a PM, but just saw this message. If I can, allow me to chime in with some recommendations.
You want a light pack, but more importantly, you want something that will be comfortable. Head down to a local specialty sport shop that specializes in hiking. They generally have more knowledgable staff than big box stores and should be willing to spend the time with you that you required to find the right pack and feel confident about your choice. If you tell them what you are gearing up for (the Camino, 800k/500mi, 30+ days, over 1 million steps) there is a good chance they have already helped other pilgrims with equipment choices. You’ll be hiking ‘hut-to-hut’ style, in the hiking lingo. That will tell them you don’t need to transport a tent, sleeping mat, require a lighter sleeping bag, not much food to speak of, etc. Overally, smaller pack and lighter weight.
Ultralight packs and packing ultralight is in vogue right now. You can find web sites, blogs, youtube videos about the whole subject. However, IMHO, unless you plan on racing to Santiago, a few extra ounces of padding might be worth the sacrifice for comfort. However, weight is cannot and will not be ignored if you plan to carry it the whole way. So it’s a trade off. My preference is towards a pack that might not be the lightest, but will be comfortable for me. YMMV.
Many will tell you that all you need is a silk liner, however you need to consider the time of year. Night time temps in early April at elevation in northern Spain hover around 0deg C. If you plan on staying in albergues, it’s important to note that they are not always heated, some fellow pilgrims might want the window cracked open, but you might be a cold sleeper, etc… From my research I have noted that some albergues provide blankets, others not. The Orreaga Roncesvalles web site, for example, requests you bring a sleeping back. So I’m bringing a lightweight synthetic.
MEC in Canada is a good, reliable source for outdoor gear – and even have a light sleeping bag named Camino, though I don’t know if that’s more for marketing or not. Another good Canadian brand that I learned of from another pilgrim is Hotcore. I ordered their Cooper R-7 as it packs a bit smaller and lighter than the small MEC. On her recommendation, I’m also brining a liner I already own that I can add if I require additional insulation.
You may prefer down. Down sleeping backs pack smaller, they‘re lighter, and provide more warmth for the weight when compared to a synthetic sleeping bag (or jacket). The tradeoffs are that they cost more initially, and they require more care to maintain them. Another tradeoff, and this is why I’m bring a synthetic bag, ff they get wet, they provide almost no insulation and take a long time to dry. April brings with it spring showers.
Hope some of this helps.