We are planning on taking this tour in 2019. We took the Greece Tour last year and loved it. 2 questions 1) is it discernibly warmer/hot in Sept. vs. October? ... which month is overall better for touring? and 2) the Spain Tour’s level of activity is rated as lesser than the Greece Tour. The Greece Tour could have been rated at the top of the scale. If you have taken both of these tours, how does the Spain Tour compare to the Greece Tour in this area? Is the Spain Tour more strenuous or less?
If you're satisfied to compare average monthly temperatures, you can go to each major city's Wikipedia page and look at the weather chart. I just checked Seville's, and October's average high temperature is 10 degrees (F) lower in October than September's--85F vs. 95F. For that city, at last, there's a very big difference. However, October is a lot wetter than September.
We have been in Sevilla twice in October and it was lovely, not hot, perfect weather for walking around cities.
We were just in Andalusia, November 3 thru the 12th. We enjoyed the weather, about 65 and sunny. Just keep in mind that averages are just that and there is a lot of variation from averages just like it can be 55 in October as well as 75. I really hate sightseeing when I'm hot. I'd rather bundle up. You might prefer hot over cold. This year October and November have been pretty wet in Andalusia. I just happened to hit a dry week. According to my weather app, its rainy again there.
We traveled in Spain last year from Madrid & Toledo south to several cities including the coast. The last week of September and first two of October were wonderful sunny weather every day. I did pack a cotton dress & a cotton top that I wore a lot to stay cooler than wearing polyester because it was warm (I'm from the Pacific NW). We had our own itinerary, so I can't comment on the RS tour level of activity.
Any tour starting last 2 weeks of Sep or first 2 weeks of Oct. Earlier in this period if you prefer some warmer weather.
Last week in September through first two weeks in October should be good. Earlier may be too hot in Sevilla and the south (Andalucía).
If you loved the Greece tour, you should be fine with the Spain one. I would not expect you to consider it more strenuous, depending upon how fast your guides are. Some walk pretty fast between sights, but maybe yours did in Greece, too. I only had to run to catch up when I found myself distracted enjoying the people, architecture, and art, or peeking into shops and smelling the aromas of food as we passed restaurants.
Late September into October should be fine unless the region is going through a heat wave like it was the last time I was there.