I would say just do wrist/hand stretches when you're dead in-game. Most likely you're pressing down on a nerve if you're also feeling it in upper arm so try not to put too much preasure on your wrist when you play.

i would also get my armrest as close to the surface of my desk as possible, doing this /¯ with your wrist/hand is way more stressful than just having it flat in my experience

I have severe carpal tunnel on both of my wrists, surgery is most likely needed. I went through a rabbit hole of egro mouse, keyboard, etc setups. I cannot stress enough that you want to keep your wrists leveled and minimize movement as much as possible.

Here is the secret sauce that I've found.

Keyboard:
A split keyboard is a game changer and you won't go back. With standard keyboards, you're always shrugging your shoulders and your wrists are cramped. With a split keyboard, you'll be able to keep your wrists leveled and you can adjust it however you want. The flexibility is nice. It takes a little bit to get used to. Fair warning if you don't know how to type properly, it will be harder to start. I use the tilt and it's perfect. Highly recommend.

Kinesis Edge - https://gaming.kinesis-ergo.com/edge/

Mouse:
Avoid trackpads. They aren't good for your wrists, it's all wrist movement. The vast majority of mice are also terrible. Since I program for a living, I use a trackball mouse (for work) and a vertical mouse (for gaming). Even when I'm not gaming, I use my trackball mouse. You have little to no wrist movement. With a vertical mouse, you'll be forced to move the mouse with your arm instead of your wrists. Takes a little bit to get used to.

Kensington SlimBlade Pro - https://www.amazon.com/gp/product/B0BLRQ4116/
Logitech MX Vertical - https://www.amazon.com/Logitech-Vertical-Wireless-Mouse-Rechargeable/dp/B07FNJB8TT/

Bracing:
Depending on how much pain you're in, you'll want to brace in different ways. Not all braces are the same.

Daily - Whenever I'm on the computer, this is what I use. It helps reduce my movement even more and feels nice when I'm having a bad pain day.
https://www.amazon.com/gp/product/B0CJJ4JR91/
https://www.amazon.com/gp/product/B0CNDBDZBW/

Sleeping - You would be surprised how much you move your wrists. If you have a bad night with how you position your wrists, it will suck very hard that day.
https://www.amazon.com/gp/product/B07BK53DH1/
https://www.amazon.com/gp/product/B07BK4J9MK/

Physical activity - You'll most likely be using your arms/wrists. It's important to brace so you don't turn it the wrong way and injury yourself.
https://www.amazon.com/gp/product/B0CD2M714B/

My case is extreme and the worst of the worst. You might not need to go this deep but this is what I've found to help me and reduce pain significantly.

Feel free to message on Steam if you need more info.

I'm very attached to my Cooler Master MM720. Out of all the mice I've used with good performance for gaming, it's the only one that doesn't aggravate my wrist pain. Typically shaped mice, especially narrow ones like the G403, force you to clamp your hand shut vertically with your wrist rotated in order to hold it properly, especially if you use a fingertip grip. That's a very unnatural position for your hand to be in and is guaranteed to cause problems eventually. The wider shape and dedicated ring finger rest on the MM720 feels much more natural to me.

I use a vertical ergonomic mouse at my work computer, but using them for fps aiming never felt right to me and most of them have bad sensors and input delay.

All of tragic's advice is good, too. Especially wearing a specialized brace when sleeping, that made a huge difference for me.

hey just to know how the sensor is on these things or if that even matters, I'm starting to get pains and occasionally around once a month I'll really feel the pain in my forearms. I have an ergodox ez that helps with typing but i haven't heard enough about good vertical gaming mice to really make a move on things yet. Curious to hear you experience.

On paper, it's not comparable to a standard gaming mouse. I'm pretty sure the polling rate is 125hz or so. However, it doesn't matter to me relative to being pain-free and being able to enjoy gaming.

TLDR; there is no good "gaming" vertical mouse

I use the Trust Gaming GXT 144 and are quite happy with it. But I heard good things about the Logitech Lift, Logitech MX Vertical and Zelotes c-18 vertical as well.
Adjusting to a vertical mouse will need some adjustment in terms of muscle memory though. I can also reccomend a split keyboard.

The Trust GXT 144 as mentioned above is pretty much the only vertical mouse that has 1000hz polling from what I've seen. There's some others that exist on amazon but they are mainly just OEM garbage that have different names. Anything that has at least 10,000 DPI should hopefully be 1000hz if they don't mention in it their product pages.

I used the GXT 144 quite a bit and it's pretty decent. I found it comfortable to use and didn't take long for me to adjust to it either. There's a few downsides to it though.

1. Because of how you hold it, it can be a bit top heavy. Sometimes the mouse would tilt to one side especially when swiping.
2. The mouse feet were pretty awful. You might be able to rectify this with some after market skates. Dot skates would be best, just make sure they are around 0.7mm thick.

Outside of that the only thing you can do is to actually look at your setup in general.

Everything from desk height to in game sensitivity can cause pain when playing. Sometimes things like tension whilst playing can be a key contributor as well. There's so many factors that can cause pain that it's best to find out what the root cause is as you might end up going down a rabbit hole and still find no proper solution to your issue.

this is all very good advice. my symptoms aren't nearly as severe but ever since switching to a logitech MX Ergo for regular desktop usage and just switching to my deathadder when im gaming seriously (and trying to be mindful of how long ive been gaming for) i very rarely experience severe pain like i used to. theres the gameball and gameball thumb if you want to go that route as well but i have no idea about them. that's probably about the fastest and easiest (not cheapest) advice, stop using a regular mouse

go see a physical therapist and they will give you accurate advice about what stretches to do that will actually help, what is actually going on etc.

Also forgot about another mouse that exists. This one I'm currently reviewing: https://www.keychron.com/collections/mice-collection/products/keychron-m6-wireless-mouse

Good sensor, 4000hz version available. Around 78g.

It's designed similarly to the Logitech MX Master mice that are often mentioned when it comes to more ergonomically friendly mice. But the M6 is actually able to play games with.

Don’t use thumb mouse/gameballs either. You’ll eventually develop pain in your thumb.

ime, my rsi wasnt helped by stretching my hands/wrists at all. I tried many different mice, seating positions, chairs, grip styles... a whole body approach worked best. my shoulders, neck, back, and chest being locked up was the actual problem. as well as unfreezing my hips/psoas so I could sit more comfortably for longer

anything that causes systemic inflammation like alcohol, excessive sugar - avoid if experiencing any pain. there are a multitude of tiny things that can cause nerve and muscle pain related to diet; lack of magnesium being a huge one, lack of glycine (collagen-producing) being another one.

I play jazz guitar, play slippi almost daily as a spacie main, and spend a lot of time on the pc, and my pain has been reduced to an almost non existent level. also, listen to your body, it knows when to stop