A recent study from the Harvard Medical School offered some additional insight to this problem. They used splinting to test the idea that wrist motion is directly linked with functional ability.
Subjects were 45 years or older and right-handed. The choice of age was intentional to mimic the same ages as patients who need wrist surgery for arthritis. All participants had normal motion and function with no history of arthritis.
While wearing a partially restrictive splint, wrist and hand function were assessed and compared to when wearing a fully restrictive splint. Although the subjects with less motion also had less function, their function was not as limited as might be expected.
Fusion does have functional effects on modern day activities. But most patients find ways to perform tasks despite highly restricted conditions. Having full range of motion is always preferred. But sometimes fusion does provide relief of pain with acceptable function.
You may want to try a simulation of sorts. Wearing a wrist and hand splint that restricts motion while painting may help you see how much you can adapt. Resting the joints with a splint may help reduce your pain levels as well. A trial of this type can be very informative before having a fusion procedure that can't be undone.