I'm glad you've found this travel community and that the others have been able to help you. I thought that NYT article was astonishing....29,000 spots out of 30,000 gone. I knew there were lots of signups but didn't realize there were that many. To me it's a testament to Rick and his company and how ethically he handled things in 2020.
Anyway...as to your inquiry on doing back to backs. I've done several. For myself, I've enjoyed them more if they are different cultures. One of my favorite combos was the Best of England and Paris and the Heart of France. Easy to transit between London and Paris but 2 very different cultures, cuisines, sights.
I did Villages of Southern England with Best of Scotland as a back to back and TBH by the time I got into Scotland I was pretty burned out on seeing castle ruins. I felt my experience would have been better if I'd not done those back to back.
My first back to back was Best of Europe 21 days + Best of Paris + Village Italy and that worked really well. The 21 day tour was a whirlwind. Best of Paris was settled in one hotel the whole time. Village Italy was out of the big cities and into the countryside so another view altogether. Very fun combo.
I'll also say that the begin dates for many of the tours remain the same year to year. Some begin on Wednesdays and Sundays, some on Fridays, etc. For instance, in looking at the GAS (Germany, Austria, Switzerland) tour, it starts on either a Saturday or a Tuesday. Then ends on Friday or Monday. It's pretty easy to then look for a tour that might start on say a Sunday or Monday to give yourself time to transit and have a day or two off between tours.
Which ones sound like they would interest you?