Me too, and I don’t even have hand pain. As an altoholic that plays multiple classes and specs I can’t be bothered to learn more than 8-10 keybinds.
Sometimes I use modifiers (though usually no more than 1-2 keys per class), for healers I use help and harm macros for their healing and dps rotation (before that I used to put the dps rotation on the 2nd action bar and shift scroll to switch between them, but now it allows me to switch between allies and enemies much quicker).
I have carpal tunnel syndrome and i cannot play anything but warlock (sometimes boomie too, you just press what shines). I cannot imagine playing outlaw or fury, i’d end up losing my arm lol. I strongly recommend playing warlock, spri or boomie. Avoid classes where you have half second gcd like rogues, smashing the same buttons like fury warriors, dot or pet specs (not bm you spam a lot) are more comfortable. Destro has now like 3 buttons lol, pruned to hell. I don’t know about healers
Preferring to use the 4 most ergonomic and accessible buttons to non spammable keys seems like a preference that is definitely not helping your ailment, quite the contrary.
I work in it, and do the best I can to improve my working conditions. Less wrist movement, few deep clicks, and resting hands to avoid hand pains and cramps. #notGettingYounger
Now I simply run my base rotation with my mouse. If not having to move, I can even drink with my non mouse hand while playing.
Balance your clicks between right and left hand, best advice you’ll get - and blizzard changing the game won’t help this, if you still have 80% of your clicks on your keyboard hand.
Another approach is playing without mouse.
I have a few friends raiding at CE level playing without mouse to, to avoid stressing wrists and better spread clicks across all fingers.
This is the equivalent of going to a football match and demanding everyone plays with their feet tied up because you have only one leg.
Most classes have at most 3-6 abilities as core rotation. With two of them being spenders at most.
Healers often have more buttons to press than kther classes, so healer in retaul aren’t actually thay relaxing anymore.
Proper keymapping is the key. Although there are classes that have a rather high apm.
At most i reach for key 4 and shift on my keyboard, the rest is done with the mouse as the buttons are easy to reach, with my thumb. While 123 or hard to reach on my mouse.
Although i can’t stress enough how a mouse with a minimum of 12 extra buttons can mean.
It’s well worth the investment.
And a little light hearted side observation.
Your post is literally the meme of retail wow being too quick for your crippled boomer fingers 
Why is it people demand the right to play?
Just like some people cannot run marathons, some can’t use a keyboard very well.
If they cannot adapt to the game then they should stop playing, instead of trying to make the game adapt to them.
I have asthma, therefore all playing fields should be reduced in size significantly so that I too can play sports without running out of breath.
Never gotten the button fetish people have in mmos.
We only got 2 hands with 10 fingers, with one hand preoccupied with the camera leaving the other 5 for movement/abilities.
Also really wish blizzard would finish their implementation of native game-pad support. It’s so close to be finished but almost seems dropped?
Perhaps Evoker may fit your play style? They don’t seem to have as many buttons as other classes, and with their main DD ability (Disintegrate) being channelled, you’re taking a few breaks between key presses
because people wont spend money on games they cant play
the awesome success of expansions after WotLK and second time after Legion show how more complexity made wow more popular, oh wait…
i also play one handed m8 (accident on the job) lost the use of my left hand so now i use the Corsair simicar pro mouse it has 12 buttons on the side + the mouseweel and if you are able to use a modifier you can double that, and about a class/spec i would go aff lock rather simple rotation if you use that mouse (Corsair Simicar) no need for modifier more than enough buttons so you can play single handed like me.
ps: BM hunter is also an option.
This is exactly why I switched from Fury, which I mained for several years, back to lock. Destro is pretty easy going even with my problem right hand and loss of some function in left.
The Logitech G13 is the best gamepad out there, with 21 nicely spaced buttons and a mini joystick for movement.
I pair this with the mad catz MMO Mouse, unlike most other MMO mice it doesn’t use a small grid of twelve buttons, it has a thumb wheel with a five way hat switch and a further six buttons. All nicely spaced around the device.
Both of these you’ll now have to hunt down on second hand sites but they’re still easily available and between them you have almost 40 odd well spaced buttons before modifiers or even touching your keyboard.
I don’t know exactly what your hand issues are but hopefully these will help. Compression gaming gloves may also make things better.
Best of luck.
They can’t really change the classes based on a handful of people having trouble with their hands but you might want to look into alternative ways of playing, not involving a keyboard, it will suck until you get used to it but it kinda is what it is I think :{
Have you tried foot switches. They mimic a one key keyboard and can be programmed to what you want. Not a total solution but one or two of these can use assigned to divert some of the heavy load.
I like high APM gameplay its fun but also tiring.
I use the keyboard mainly,aswd and the spells from 4 to 9 so my hands don’t overlap
I also use a elcom huge gaming trackball, no wrist strain it’s awesome.
You could try demo lock, it’s not that fast and not many active spells with the right build.
Most specs if you choose the passive options and play it well it’s not much different to active ability maybe 4 to 5 % different.
Not trying to be that guy… But did you ever have considered playing a different game with a type of gameplay that doesn’t hurt your hands?
There are several game genres out there, many with thousands of playing differently games. Sometimes the best solution is to take a fresh breath of air.
I use WASD to move and Spacebar. Keys 1-6 and everything else I click with mouse 
i already play each expansion for a few months til our group gets bored and leaves, after the turd that was bfa and shadowlands i wouldnt have bought DF if i wasnt already subbed for wotlk and df wasnt so hyped. But I do want to play those few months and i pay blizzard AAA price for an expansion so i want AAA experience.
from time to time they do fix issues, dragon flying fixes old flying being boring, removing double tap reduces bloat, but sometimes they fail, like new talents they made to make more choice and customization didnt do that
I can see that point. But it is not Blizzards responsibility to fix your hand pain. That’s yours.