You should be able to set Cortana as your default voice assistant. Are you sure you have the focused inbox turned off in Outlook? If it is, and it still doesn't work for you, you might want to try Nine. It is kind of expensive, but does have a free trial, and the email, calendar, and contacts integration is great, as are the widgets.
The UI is what I missed most about W10M, and ultimately what brought me back. I definitely understand what you are saying about the banking app, which is pretty much the only app I miss on W10M compared to iOS and Android. It is also frustrating how much faster OneDrive works on both iOS and Android. When uploading large files I find my W10M and even W10 apps struggle, usually resulting in an error, but iOS and Android have no problem.
If Onedrive could integrate into pictures on either iOS or Android the way it does on W10M, I might be even more tempted to switch. I know I could migrate all my pictures over to Google Drive or iCloud, but I like OneDrive, and I have 1 TB of storage. I have all my music stored on there too, and using my sign-in info my wife can listen to it all on her iPhone too with Groove Music. It works great! Maybe I could do the same with the other storage options, I just haven't looked too far into it.