Your experience is similar to mine. My wife and I found that our progress improved when we began taking solo private lessons in combination with lessons together. Each week we currently take one lesson together and my wife also takes a solo lesson with her male instructor with whom she also participates in pro-am competition.
I am rather more competitive. I take at least three private lessons each week plus any group lessons I can find. Some of the time I take solos with my female instructor with whom I too compete pro-am, however, I have an amateur competition partner We take private coaching sessions as often as we are able.
My regimen may be considerably more strenuous than you are interested in but I can assure you that the rate of progress is much, much greater. While I am now rather more advanced than my wife our partnership has improved tremendously and for several reasons. She learns a great deal in her privates as do I so we can use our lessons together to work on partnership skills more than new figures. As a result of all the work I do I am much more skilled as leader in partnership with my wife.
You probably need to define your goals then determine the amount of time and money you are prepared to dedicate to achieving those goals. Should you and your partner decide to take solo instruction in addition to your current lessons I cannot imagine any studio being unwilling to cooperate.
What ever you decide to do, make sure it remains enjoyable.