Oh sorry I didn't elaborate on the source of the pain. My best guess is that it's a combination of many factors, especially these:

1 - broke my left arm (around the elbow) as a kid and didn't have it in a cast so it's very crooked and although a specialist said it's just a cosmetic issue, I beg to differ because in some specific exercises/movements, it's not smooth or in the worst case I can't do the movement (I can't do push-ups, every other exercise is do-able except for this, it's like my left wrist/elbow just blocks the movement)

2 - as a teen I played a lot of volleyball for a couple years which is one of the worst sports regarding injuries, literally all of my colleagues had at least 1 long-term injury (hand/wrist/feet/ankle) so I'm surprised I don't have one but my fingers are significantly more crooked and it definitely strained my arms in general

3 - I've been playing on the guitar for over 10 years and during those 1st 8 years I spent all of my free time practicing, which is also a huge strain on the left hand, and after like 5 years the wrist pains started so this was probably the last straw lol

4 - Even though I started drawing seriously/more often only after the pain started, I have struggled with holding my pen wrong (as a kid I self-learned to hold it very tightly between my middle and ring finger) but thanks to taking it seriously and researching, I've learned to hold it in a more sustainable and efficient way (otherwise I wouldn't be able to control the pressure) so I don't think it's a factor anymore

Regarding what I use when drawing in various ways, I do both large and small (from 1:1 paper for figure drawing for uni up to A5 sketchbooks), use my whole arm (though with Drawabox I'm learning to use my whole arm for smaller scaled strokes and on a desktop instead of an easel for life drawing) up to the fingers, use the writing grip mostly and the overhand grip for larger/smoother lines or curves.

Oh we do have a powerball in the house, haven't used it yet but I'll give it a go to see if it adds more support! As for PC usage, I do spend a lot of time on it, but I've never had any issues so far, except for exam season where I'm writing a ton of notes or an essay/thesis for which I arm myself with multiple braces, definitely recommend if one can't exactly afford to rest, it's doable in the short term.

I only have a gel mousepad and rest my arms a lot as my chair's arm rests are at an ideal level when using a keyboard (in the under-the-desk drawer) but in the near future I'm definitely gonna buy a vertical mouse and I do dream of training my non-dominant hand to write and draw.

Thank you for all your points and tips, it's always a plus to know more and be more aware of things one already does.