goldwing is the best bike for you I think. since it's heavier than most bikes.

prior to my first and current big bike (weighs around 240kg) I rode ninja 150rr also. before I got my suzuki, the shop lent me a z750 for a week. I have no problem getting it around even though I could not flat foot during a complete stop. er6 is quite a slim bike & as long as you are 165cm or more, it should not be a problem for you to handle it. I'm assuming the riding school provides er6n as their learners bike.

since you currently ride the ninja 150rr you already have the basics to riding big bikes. once you got the balance of the big bike, it's actually not that easy to drop it.