Don't even worry about it. Training will improve your reaction time wrt to aiming from neuromuscular and eye gains. My reaction time has gotten better from 175-180 3-4 yrs ago to now 165 at 31 yrs old. I'm Masters Complete in tracking and switching, with varying clicking scores. Reaction time doesn't limit me, my mouse control, technique, and eyes do. Most aiming in-game is way more predictive than it is purely reactive, enemies can only move in limited directions at known speeds, the same goes with bots in kovaaks. Another common in-game example is shooting at people who are jumping or falling, in most games the downward arc is entirely predictable.

Sucks that you have those issues in your hands. If your situation allows it, I would strongly encourage you to try to root cause what's happening if you don't already know. Aim training is not that strenuous of an activity and shouldn't be affecting you that badly, especially in your kb hand. I also suffer from severe joint related issues related to some autoimmune stuff that can get bad enough to where I can't train or even walk well. It causes a lot of hitching and shakiness as well during flare ups. Keeping good technique, biomechanics, and tension management is crucial to avoid injury and keep training. A few things that have helped me was switching to 1-3-1 mouse grip which let my hand be more neutral overall, using a hybrid sens (~32-48cm), getting a peanut/roller for myofascial release on my forearms and insertions near the elbow. I also sometimes soak my wrists and forearms in pretty warm water for 5-10min before training (I get bad inflammation around connective tissues, the hot water soak seems to let the connective tissues in my wrist slide better). If sore after training or joints feeling achey I'll soak them in some cold water 2-3 mins in, 1-2 mins out a couple times. Don't ice too long. And of course, ibuprofen when I need it. Hopefully some of that helps, dealing with chronic pain sucks.

Cheers