Thank you Ted, these are precious recommendations!
I think after all it will be a combination of methods that will make the difference.
When I first encountered that problem back in 2006 (Well actually I know now that it had actually started long before, in 2000 already) I was in panic, searching for a different job, ready to throw everything away. That isn't the case anymore, although I am at the time experiencing a deterioration of the symptoms. Since years they occur also when I'm not playing at all, in rest, while doing household chore etc. And I am not even playing a lot.
I am not anymore afraid of these injections and I feel completely safe with the person who is treating me, he is very experienced and has worked with many pianists. And I feel I need to do something.
Anyway, I am aware of the deeper cause which is a call from inside, so to say a call from my own soul which tells me to keep consequently close to my path and not to dissipate my energies with things that are not on this path...and to learn a better stress management. Because after all it's not only the excessive practice repeats who cause the symptoms, but practicing under the influence of stress, anxiety, negative emotions etc.