Very little passing in wee tyke Pop Warner and not much more after that age. Like you mention, most of the development occurs in middle school in flag or 7x7 passing leagues. The footwork and field vision learned by soccer players is a significant benefit for QB's. Maybe a steeper learning curve for soccer players turned QB, but not out of the question. A kid my son grew up playing soccer with, moved to football country in middle school and started playing non-contact football. He had Power 5 D1 offers by the time he was a sophomore. It's not as common for a soccer player to play QB as other FB skill positions, but it's not uncommon. Youth soccer is still better training for football than youth contact football. The toughest part for many soccer players is learning to tackle or be tackled. My son is still not very fond of either, but that's likely due to other factors.