Have you tried compression gloves? I found gloves that are to help with knitting, help with drawing too.

I haven't tried the Copper brand, and I'm not sure if they actually help more or not, like they claim.
https://www.amazon.com/Copper-Compression-Arthritis-Gloves-GUARANTEED/dp/B00ZO4WW7Q

I found some at Joanne's, but I don't have the package anymore and no idea what the brand they are now. XD they help a bit.

Not sure if this is compatible with the model of your pen or not, (they look very similar at least) but maybe a pen grip as well as a glove?

https://www.amazon.com/Plus-Ergo-Grip-Wacom-Stylus/dp/B0111LLPH8