@Jamie Giles, if I can extend a couple of points
@Kanga has touched on, and that is if you have bad knees you need to be thinking of both reducing the weight you are carrying, and using walking poles to reduce some of the pressure on your lower joints, in your case your knees.
@Kanga and I (and many others) have contributed extensively on both these issues so you might be able to find more details on this with a simple search, but I will give you the short version of my views here.
At 1.9m, your BMI=25 weight would be around 90kg. If you use a from the skin out target of 20% (FSO20) you would have an overall weight target of 18kg. My total weight for worn and carried items (clothes, boots and poles) is about 3 kg, and I typically carry up to 3.5 kg in consumables (mainly food and water) although it is possible to reduce this to under a kg if you only carry a litre of water and no other consumables. If you were to carry similar loads, your 12 kg backpack would exceed an FSO20 target. Noting that you already know you have knee problems, a bit of work might be needed to keep your weight well under the FSO20 target. I believe that this is achievable at the time of year you are walking, and a pack weight of around 9 kg bare might be a more reasonable goal.
Poles are simple, but the effect relies on them being used properly. There is lots of good discussion on this, as well as some mis-informed blather. Its really up to you to sort the wheat from the chaff here. The results are simple. Used properly, poles can reduce the loads on your lower joints by 5-7 kg on every step. That is an enormous boon for your ankles and knees. Equally, used properly, poles can be used every step of the way from SJPP to Santiago. I have used poles on all my pilgrimage walks all the time I have been walking - from albergue door in the morning to albergue door in the afternoon. Not used properly, and you won't get the pressure relief, and you will tire out your wrists and hands. The poles will then be strapped onto your backpack and be just so much more unnecessary weight!
On your original point, I have used boots on all my pilgrimage walks, and will continue to do so. Many people walk in shoes just as successfully. I hear the discussion on not using waterproof boots in summer, and my only observation is that the only time my waterproof (leather) boots have retained a lot of moisture is when it has entered through the hole in the top!