Yea, true. I will want the photos to auto upload to one drive at full res. for example...
I'll be glad to tell you my experience, if it will help. The iPhone has been my DD with occasional switching to various Androids and my 640 on Insider for months now. OneDrive (and Outlook too) is excellent on iOS. For the most part, you can have images upload there directly instead of iCloud. I don't even use iCloud at all. It's not as instant as it is on Windows Phone but it will upload. iOS makes it easy to set other services as primary. For example, I never have to open the mail app to read mail. I get an Outlook notification and I read it in Outlook.
My contacts being hosted in Outlook is also great, because again, you can carry that to the iPhone.
Check these great links out on how to make it all work:
iPhone for the Windows Guy: Getting Started
iPhone for the Windows Guy: Use Your Microsoft Accounts
As for what you'll miss, as said, you will miss customization. iOS is the bare bones of customization. You can change your wallpaper, lock screen, rearrange your icons, and use a 3rd party keyboard like Word Flow, which will allow the swiping that you are probably used to. SwiftKey does the same, along with GBoard. That's about it. If you're anything like me, you'll miss having a phone that can be any color you want (or truly different). And no, it's not the same experience with a case. That's what people like to make you think. The only way to try to get there is with a case.
Me missing that is what has me going back and forth so much. I love how simple things work on iOS but the customization makes it hard for me to be content.
Some people here might say, oh that doesn't matter, or that it's not a big deal. This is one of those things that is very person dependent.