I AM A CREACHER MADE OF PAPER MACHE
I don't know how many artists or folks in general deal with the same issues as I do in my hands (heck nowadays they'll sting and ache if the weather itself is w e t), buT I know its taken me many years of chronic issues with both tendonitis & carpel tunnel to find what's worked for me, to get through whenever these months long flare ups come about. (Usually whenever I've gone overboard with working on/playing something I'm excited about and don't stretch or wear braces like I should. Or making a website for example :)))) )
So these are all the different resources I'd gathered for me overtime, which hopefully are of some use to you too!
Exercises & Stretches
I can't find many of the original sources for the static stretch references, as I picked them up over the past decade or more.
Do not push your body into these stretches if you experience genuine pain, and seek professional assistance. I'm not an expert on any of this, just a tissue paper goblin doin what I can to keep doin' what I love, and sharing what resources have helped me cope over the years~
(The arm and leg cool down stretches especially help me the most! Even when I fall off of using ringfit I do those particular stretches pretty constant lol)
(This vid is nice for both general stretches as well as letting anxious tension out.~)
Ergonomic and general relief thingz to buy
Many of these will be Amazon US links, though any of the products frOM Amazon are like to have equivalents on other sites too!
Ergodox Keyboard Legitimate lifesaver of a keyboard. Any kind of ergo keyboard is a good step in the right direction- but when I started typing far faR more than I already did after getting together with my partner and starting a new job, its saved me from many nights of stinging elbows and crinkled fingers. 
It is very pricy, but after nearly two years of use as of writing, I am obscenely thankful I'd gotten it. (Highly recommend putting shortcut keys for shift/capslock/tab near where your thumbs are too! Your pinkies will thank you)
Just a vertical friend for smaller hands like me, and even then it's still kinda chunky. My hands still get fatigued from too much repetitive button holding on a n y device (especially using the scroll wheel or holding right-click down), but this gets some of the way there.
Keymecher Bluetooth TrackpadA more recent purchase as of writing, actually since starting this site because my own hyperfixation of working on this- it was becoming a physical problem LOL. It's biggest strength is in any tasks involving scrolling, lots of double clicking, or dragging. So it's a huge boon for web browsing or editing if I need a regular mouse break.
Futuro Wrist BracesFor the carpel tunnel and sore muscles side of things. Their only downside is having to replace them almost bi-annually from wearing out the velcro with overuse, but I've never quite found another brace like these that are comfy for longterm use without cutting into my skin, especially around the thumbs. These alongside the ergo keyboard have been the two biggest saving graces of anything else I've bought.
Compression GlovesVery nice for when tendons act up, though with how itchy the inner lining is on several I've tried, turning them inside out is a near instant fix! I'm not wearing them for looks afterall. 
Just be aware of what size you purchase since there are a few different kinds!
You can just get the rice mitts to put in the freezer, then heat em up once the cold wears off! Going from massaging my hands against a cold pack, and then letting them rest in a warm one though, I've found to be really effective for relief.
Padded Mouse PadFor when you can't find quite the right angle to put the wrist at. Now you dont even have to think about it! I think I got this cause the cushion I used to have would still slip, or I'd idly move the mouse too far away and rub the bottom of my palm on the hard surface a lot.
Armchair PaddingJust something useful to keep the elbows from getting pinched or sore while at the desk.