I did the santiago to finisterre leg several years ago, with just a simple waist belt - the one that has room for two 1/2liter water bottles in a small pouch in between. In that pouch I had a toothbrush and small paste, a cut off piece of soap to last the distance and a silk wraparound skirt I could change into while washing and drying my clothes and later sleep in. I washed my clothes each day as soon as I got to the albergue, risking they may still be somewhat damp in the morning. I never felt I needed anything else but it was summer. I started off in May in France and walked 1200 km app in total with no more than 6 kg - backpack weight included. I didn't carry a sleeping bag, just a sleeping liner - and most albergues provided blankets - but not all, which did lead to a couple of chilly nights here and there - when someone insisted on having the windows open e.g or in an albergue under renovation without doors or windows! I also had only one of those emergency ponchos for the rain - the garbage bag types they sell at tourist destinations - and did regret that in May in France when it pelted with rain for 5 days in a row - and even hailed on top of Mt Aubrac. So, it depends, I think, not only on the season, but also your tolerance or your travel style and finances maybe (would you consider a hotel at times e.g.).
