Hi Miguel. We have something in common, 65 years old and a continuously sore left wrist since buying Sigma 150-500mm. I don't know whether you have come across this product before,
Gopro ST-173 Pistol Mini Hand Grip Tripod Support Stand Fr Gopro 1 2 3 3+ 4 DSLR.
I just purchased one and they work great. By reversing the hand grip opposite direction to a normal pistol grip get about 2 f stops more speed with heavy lenses. I tried it out with a Hanimex 300mm 1.2kg lens yesterday and was able to shoot at 250 shutter, without the grip day before could not get a decent photo at 500 shutter. I used something similar in the 1970's with a Hanimex zoom on a Pentax Spotmatic, that one had a cable release slot in it and you just pressed the cable release knob and presto, even then it gave me 2 additional f stops. No IS then just common sense.
Best wishes Ron (galelegg).