I have received some reports about NAS issues with iOS / iPadOS, but the same NAS works fine with the Nitro on the Mac. Personally, I have found that a Mac mini acting as a server works fine with Nitro on iPads, so that could be option 4 – use the Mac mini as a file server / turn on file sharing.
Otherwise, of your above options, I would agree that #3 is your best bet given that your LAN is fast, but your WAN is slow (and iCloud is squirrely).
The external disk could be okay too – it could be backed up regularly by setting up a backup script / service on your mini and periodically taking the drive back to the Mini, where the backup system would then run.