Your elbows and hind arm are pushing back the sleeves a bit, causing folds. Iīve to think about how to handle this. Iīm not sure it can be done in a one seam sleeve.
In the meanwhile you can scoop out the back of the scye seam a bit more: just baste away a bit of the seam, crescent shaped starting a bit above the yoke seam, scooping out letīs say about 0,75-1cm of the body and sleeve (keep the scye seam sewn), rejoining the seam line at the bottom of the armhole. Maybe we can rid of the deep folds there.
I wouldnīt recommend a formed two piece sleeve in a shirt. It would look too much like a blouse or dress sleeve
But you can make the sleeve a bit slimmer by making the sleeve seam lines a bit concave instead of straight.This would probably harmonize the proportions of sleeve width and body width, without making the sleeve tighter at the base line.