Just finished biking from Bayonne to Santiago and then on to Muxia and Finistera.
Here are some suggestions:
1. Clip in pedals if you are experienced and even if you are not since if you are doing the full show you will be by the time you finish! Take some with you, Shimano make a version that you can use either either way, i.e. without clipping in or vice versa. The use of clip ins will increase your efficiency significantly. If you are renting you can change the ones that come with the bike to yours. Of course you will need shoes to match which unfortunately will mean that you will have to have another pair for the evening. My eveing shoes were a little heavy and took up a lot of room. And if I were to do it again I think I would have taken a lighter pair.
2. Leggings at least in Sept. Even when the days are hot the mornings are cool if not downright cold.
3. Warm gloves, not just finger gloves. Its cold in September early morning at first light. One morning when I left with the next cafe con leche stop being Fromista 20k away it was very misty and by the time I got to Fromista I was sopping wet and my hands were freezing cold (that why its called Fromista (lol)). Bought full length warm gloves at the next available opportunity and of course never needed them again
4. Rain gear, pants and jacket. September this year was outstanding from a weather perspective so only used them once or twice while I was riding but the year before there was a lot more rain.
5. Biking pants that come with inners with padding. Make sure the pants have pockets with zips on/in them for keeping your phone etc. As an aside forgot to fasten my pocket with my phone in and just coming into SDC tried to jump up onto a wooden ramp and went over the top. Fortunately there was a barrier there that prevented me going on to the busy main road . Later found my phone missing went back and it was lying over the barrier on the road--still functioned.
6. Finger gloves with gel padding for when its not cold
7. Long sleeve SPF 50 cool max T shirt that will dry quickly after washing-it will need washing every day since you are going to sweat a lot climbing the hills under pedal and more so when you are pushing the bike up the hills which you will do a lot!
8. Sunscreen
9. Some means of keeping up your potassium and sodium levels. I bought some tablets can't remember the name and generally watered down my wine with cans of Aquarius!
10. A phone with loaded maps of Spain and France (for the first leg if you are leaving from SJPP). So that you can know where you are at all times. On a bike especially at the bottom of a hill its easy to miss the signs (which are abundant the signs that is) and end up going the wrong way. I had an Iphone with Forever maps on it. I am sure there is other stuff that works just as good. As they say dont leave home without it or something similar. Also great for when you are touring the Cities on foot.
11. Phlyssum Husk or some other laxitive, at time you will see fields of vegetables epsecially the tall cabbage variety, however, I think all the vegetables are exported to China or some other distant country since the restaurants never server any (lol).
Never needed or so I thought lights and never had a problem and not a good idea to ride in the dark. But still good to have them.
Things that I did not have over and above anything above would be:
1 A better front bag that allowed me to see my maps at a glance without having to pull them out evertime.
2 34 teeth on the back Cassette mine only had 32 with 22 on the front. Every time I was on a hill I was wishing for a lower gear.
3. Maybe a place to put my phone on the bike so that I could see it and not have to take it out of my pocket every time to look at it.
4. A megaphone with a recording on it that at a push of a button exclaimed "Con permiso, muchas gracias, buen camino"! You will be saying it a lot and if you don't will be getting a lot of dirty looks and abuse and sometime the same even if you do say it.
Since you are going in December you might not need the above as much since there will not be as many walkers out there but you are going to need the rain gear and warm clothes even more than you would in September. And in you rain gear you are still going to sweat a lot.
Buen Camino