So if you have all those things in your day pack, only thing left to ship is your sleeps sack? I had a 16 Litre osprey. It was adequate. It held almost everything you had in your day pack: plus cell phone charger and my sleep sack. I didn't need anything else. And somethings suggested in daypack like power bar and repair tools aren't required. Needle that can take your already packed dental floss for repairs is handy. Albergues will supply if required, however.
My main pack was just under 6kgs, for the 5 days I sent my pack ahead (I hurt my knee), I took nearly 1 kg with me in a small cheap backpack I bought in Los Arcos, which dropped my main bag down to 5kg. My day pack also included some fruit and a snack.
There was a lot I took that I never actually used, for instance I took a poncho, sleeping bag, jacket etc. We had only one days rain in a whole Camino, and it was so hot I never wore my jacket, or even took my sleeping bag out for the first 3 weeks.
And I didn't take guide book, toiletries or first aid with me in my day pack, as I was walking with someone else and I borrowed a bit of toothpaste off her if mine wasn't there. I was fortunate not to get blisters.
The next time I went it was cooler to begin with and I used more of my gear more often, in fact I actually used everything I took.
It shows though how little we would need if we could guarantee the weather.
A couple of points though that I didn't make in my earlier comment:
It helps if you get a day pack that allows access to a drink bottle, mine didn't and I had to carry it in my hand. Though it did mean that I drank more frequently.
And to spray your main pack, it will be travelling in a van in close proximity to a lot of other backpacks, and it would be easy for a bed bug hitchhiker to relocate to yours.
The hitchhiker thought didn't occur to me until after I got bitten (I had sprayed my pack anyway)