So I did the shoulder mod on the SH Figuarts mark 50 and it looks so much better now. The arms no longer flare out and can actually rest on the side of his body now.
Here's an explanation on the mod:
http://thefwoosh.com/index.php/2018/07/bandai-s-h-figuarts-avengers-infinity-war-iron-man-mark-l-review/