I realize the OP asked about group tours with his wife, but this thread has drifted enough that I figured it would be OK for me to chime in as a solo traveler. (DH doesn't particularly like to travel, and my friends are often not available for time, cost, and/or heath reasons. So if I want to travel, more often than not it's on my own. And while I'm very self-sufficient, I'm reluctant to travel entirely on my own, especially overseas. )
I've found sort of a happy medium for group travel.
I'll choose a tour that departs from a location that's particularly interesting to me, and I'll book my own accommodations a few days ahead to explore that area on my own before the tour starts.
I also now try to find "women-only" tours. I've found those tours tend to be somewhat more accommodating to solo participants. And, I've observed an odd dynamic on a few of the co-ed tours I've taken on my own. On each of those tours, there was a couple with a very gregarious husband and a less outgoing wife. The husband would often chat with other women on the tour -- nothing inappropriate, just friendly -- and over the course of the tour the wife would become downright hostile to those women. I've seen this happen on three of the five co-ed Road Scholar tours I've taken. It was a shock to me in the first instance, especially since I was one of the main targets of the wife's wrath, but on subsequent tours I could see it happening and I would simply find it amusing. I subsequently took a Road Scholar women-only tour which didn't have any of that foolishness and was great fun (and yes, I realize that dynamic could exist with same-gendered partners, or with a pair of woman friends travelling together, but I simply haven't experienced it, at least not yet).
Although tour groups can generally room solo travelers together to minimize costs, I always get my own room because I'm not willing to risk ruining a trip by being stuck with an unpleasant roommate, and I can afford it.
I'm currently eyeing a tour to Denmark, Sweden and Norway next year with a group called Women Travelling Together (WTT). I intend to spend a few days in Copenhagen on my own prior to the start. I used to travel to Copenhagen for my j*b but I haven't been there in about 20 years so I'm eager to revisit some of my old haunts before I join up with the tour group. I haven't been on any of this company's tours and I'm eager to check 'em out - the tours generally have only 10-20 participants which is a bonus.