Suggestion for fitting arm holes is to keep side seam open, fit the sleeve shoulder seam, then sew the side seam and under arm seam in one action. A lot easier than fitting the sleeve into the arm sythe. 😘
Ah my bad, lost in translation. I do the shoulders exactly like you suggest. The problem was the lower end of the arm where the hand sticks out. I now do them also like you suggested, with the side+underarm seam being the last one I close on the feature