In 2019, I walked the central route from Porto, but talking to others who walked the coastal route at the same time, weather conditions were similar.
I left Porto on 26 September intending to follow the stages recommended in the
Brierly guidebook. After a few days, it became apparent that I would arrive in Santiago 3 days before my flight out. I preferred to spend more time on the route so changed my plan, added a day and stayed at Albergues in between the recommended stages.
At every stop, I found a bed at the first hostel I tried. Up till Tui, there were few pilgrims, at times, I could walk for 30 minutes or so without seeing others. The numbers did increase in the last 100km, but I only found myself walking in sight of more than a handful of runners a couple of times.
It did rain overnight 3 times which meant walking on wet roads in the early mornings. However, it only rained during the day on the last day 8 October when I walked from Teo to the Cathedral in Santiago. That was the only time I walked in the rain, but it was not cold.
The lowest morning temperature was 9C, the highest in the afternoon was 27, most days were between 14 and 23.
I found it an ideal time to walk.