Hi. I mostly use a pendulum serve, but like to mix it up with bh serves for variation. My bh serve is not very spinny with more underarm and less wrist, but I manage to keep it low and just almost half-long. I get comments on that it is good anyway. Some also expect more spin and make mistakes. Also easy to add subtle variation with nospin/side/topspin. It should be possible to have a considerable amount of spin on the backhand serve, even if the wrist is more restricted in this position?
Glad your thinking a lot about your serve variations as this area of the game is key. I agree also that its not always about the amount of spin you produce but the deception, depth and the height of the serve. If you practice the backhand serve often you will be able to get a lot of spin without to much wrist. However, I think it would be good to experiment in the training hall with using a bit more wrist as you will get a lot more bat speed, thus more spin. Having more focus on the wrist will make the contact more fine and subtle to, which will increase spin and deception. Overtime you will be able to find the optimum balance between less wrist and more forearm but for now I would add more wrist. You will be able to make your serves become two bounces a lot more often by using your wrist a bit more.
If you can send in a video to us on yourself doing the backhand serve that would be great as it would enable me to see how much forearm your using in comparison to your wrist.
Happy New year Martin look forward to hearing from you.