I can handle PC - PC by simply using dropbox and setting up file system symlinks. My issue is the PC to PS4
This was implemented via save server slots on the login screen in FFXI, and had no issues. I was limited to only a few slots and could only be accessed from the character select screen.

The issue with the thumbdrive is that sony requires the game data to be encrypted on media (so SE claims in 2103), so any option would have to be done in-game.

I suggested letting us link our dropbox accounts to server as the server / storage between the two consoles, but honestly storage is cheap and 6 TB isn't a lot for that many users. a 6 TB datacenter grade Western Digital HDD is only $250 USD and would cover all of the users in your example. If even 10 of those people feel like I do and bought the PC version, they would make up that cost. Also my PC character data is only 630Kb... so that number definitely isn't for every user