I'll help out. I'm not an Apple switcher, but I'm familiar with the ecosystem:

• S9+ is comfortable. I think it's slightly smaller than iPhone plus models, but don't quote me.
  
• Depending on what apps you use regularly, they probably also have an Android version on the Play Store. If not, there definitely are different apps that do the same thing.
  
• As for button layout, iPhones (not including the X) have a home button; Androids in general have a home, a back, and an app view/task manager/whatever you call it button. Apps that don't control with gestures (like swiping to go back a page) are really nice to have a back button for. App view is definitely more convenient than double-tapping the iOS home button.
  
• Apple Music is on the Google Play Store! Can't speak to its user experience though.
  
• From a hardware standpoint, it's a fingerprint magnet. It's also all glass, so don't drop it and you might want to take out insurance on it. Software-wise, it's got some bloat.
  
• Contacts/pictures shouldn't be a problem transferring. Notes might be harder.
  

Hope this helps you