Many, many years ago, I learned that your forearms should be parallel to the floor when playing the piano or keyboard. Not only does it save your back, but also takes a lot of the pressure off your wrists.
Kaarlo, I am just 78 years of age, but I have a very bad back, three herniated discs in my lower back, one in my neck, and nephropathy in my feet. I tried standing for about two years while on stage, then realized that I was exacerbating the problems and the pain was excruciating. That's when I went to the chair with an adjustable foot rest, seat height and back rest - problem solved. I continued to play for another 20 years before my body gave out. I would bet some big bucks that your physioterapist spends more time sitting than she does standing.
And, I am surprised that you are not using a headset mic. The problem I, and many others, encountered with a boom mic is that it is never where you want it to be, especially if you are dancing around the keyboard while singing.
Good luck,
Gary