What do you think about signing up for a highlights tour that hits many spots for a day or two when you're a veteran of independent travel and have seen many or most already? Think it's worth repeating some in order to travel in the group, and just vary it where you can with side trips or walking tours during free time? Ever done this?
Will you be bored on this tour, since you're a veteran independent travelers and have seen most of the sights already?
I think it would depend on whether the group offers you something unique, or if traveling alone is becoming more burdensome.
I did that last year, combining 10 days in Italy with a group from my church, with about 2.5 weeks on my own. I've been to Italy number of times, always solo, including almost all of the places visited with the group.
For me it was worth it because of friends in the group, the spiritual guidance my pastor provided, and the opportunity to attend an ordination in St Peter's Basilica (which was amazingly beautiful). For my favorite city, Florence, I went to Mass with the group and then took off to do the things that were important to me, skipping their tour. That worked well.
If I didn't have a special tie to the group, or it wasn't offering something unique, I probably would not join to visit a locale that I know well. If I wanted to join a group under those circumstances, I would opt for someplace I had not been. For example, I'm thinking of going to Turkey with a group because I don't particularly want to deal with the effort of doing on my own.
What would be your reason for wanting to go on a tour of places you've already seen? If you can post that, people can give better answers.