The only places where storage is a problem is with photos and backups. Everywhere else, apps can plug into the Files app and offer seamless integration with other apps and the file system.
I really just wish they would expand the backup functionality to enable iPhone/iPad to generate an encrypted backup package and let you do with it what you will. Upload it to a service, copy it to a local host. But without needing to have a Windows/Mac on hand to do the heavy lifting. If the phone can self-manage its backups in iCloud without a PC or Mac in between, it seems reasonable that it should be able to do so with other storage providers.
Photos is really the place begging for improvement, though. I really like having my entire photo library available and searchable on my phone, but not having to carry TBs of photos with me. The iCloud Photo Library optimize mode works perfectly for this. It's just frustrating that if you opt to avoid paying an arm and a leg for iCloud storage, you have to manually manage your local Photo library on each device and make sure that whatever third party app you use to backup and manage photos is picking up every photo before you delete it.
They really just need to make the Photos app work like the Files app, where third party photo libraries can plug in and integrate, so that as new photos are captured they can be uploaded to the libraries of your choice and "optimized" away from local storage once they've been uploaded.
