SL,
I faced a similar dilemma. For me, I began to realize that I wanted more than I was getting from the group classes I took faithfully every week. I felt I couldn't afford both group and private lessons. I study ballroom through several media venues like you, and came to a point where I wanted to progress beyond what the group was doing. So often, group lessons aren't tailored toward moving students up through the levels of dance, and most of us can't each ourselves the finer points without professional intervention. You seem to be aware of improvements you think you need to make in your posture and frame. That is a sign of awareness of what you have learned. I think many students never reach a level of awareness that hints to them they they might need to move forward. And many have no desire. Your tone suggests you have the desire. I am now taking private lessons with an excellent teacher. I can't afford the instruction as frequently as I would like, but with practice, I am improving over what I was getting from the more regular (and cheaper) groups lessons. My instructor is correcting and improving me with each lesson, and I feel that I did the right thing switching. She understands I can't afford a lot, but she appreciates my dedication, passion, and practice.
Give it a try. You can always go back. Just keep dancing.