I've toyed with this, too. And while perhaps not ideal, my approach has been to use all 3 of the following depending on file size, whether or not it's a sensitive file, and what device(s) are involved:
- NextCloud storage for large files
- Created 2 Session accounts in order to create various group chats for transferring various files between Android, iOS devices, and to/from a shared iPad
- Created Delta Chat groups (only 1 account needed) for transferring files between PC/Android devices and between user profiles on GOS device. (I personally prefer not to use Delts Chat on iOS devices, so I use Session instead with those.)
Delta Chat uses your connected email service account storage, so be sure to use the message auto delete settings to avoid duplicate files from multiple user profiles/device. For example, I delete all messages from all users except main user profile after 1 hour.
Example group chats: "Graphics", "Contracts", "Quick Notes", etc.
To clarify, I use Delta Chat for myself only. I use Session for iOS devices, shared devices, and for sharing files with another person.
So, largely speaking, it's a lot of sending messages to yourself, categorized into different group chats. 😎