I don't know if it's the OS or the dithering/gpu used. The only mac I can use 8h/daily whit minimal strain is the 2016 Macbook Pro 15 whit Intel HD 530. I bought it new and since then I had no strain whit any OS whatsoever, I tried changing it a couple of times but I never had any luck. Tried multiple M1/M2 also and instant strain or at most in 1-2 hours.
You should give it a try.