Thank you - this is very interesting.
Although I am a (casual) iphone user (my iphone has never seen my real name or my real phone number and has never touched rsync.net) I was not aware of the user-visible filesystem nor was I aware of "Goodreader".
Does this user-visible filesystem allow me to just copy over my entire music library (which is files and directories, and no knowledge of apple/itunes/ios) and then let itunes browse it, locally on the phone ? Or do I still need to do complicated import tasks ?
Sort of. Each app has the equivalent of a folder. You could use the Files app to copy a directory tree of music files from a remote server, into the folder of a music-playing app (e.g. Flacbox, Nplayer). Then all the files are "local" to the app. This assumes the copy doesn't fail, e.g. several GB over a flaky network, as the Apple-native Files app can't resume a copy, and will abort on every duplicate.
> and then let itunes browse it, locally on the phone ?
The on-device music app from Apple conflates downloadable iTunes-purchased files with streaming Apple Music. It does not play local files AFAIK. But you can use several third-party apps to play music & video.
The challenge is keeping the device and remote server in sync :) Hence GoodReader, which does a decent job of file synchronization within the constraints of a non-Apple app. It can also play music and videos, but doesn't have all the bells & whistles of a dedicated media app like nPlayer. But then those apps don't have file sync.
While apps (e.g. media players or editors) can edit-in-place or view-in-place files within other apps (e.g. storage providers like GoodReader), this doesn't seem to support the ability to maintain a continuous mirror view / symbolic link on a directory tree from GoodReader into nPlayer.
Other apps for an iCloud-free iPhone: 2Do (CalDav sync), local scraper/search DevonThink 2 Go (Webdav sync), Secure Shellfish (local access to remote files via SSH), HereWeGo (formerly Nokia offline maps), iCatcher (podcasts), Codebook (password manager with native sync to Mac), Kiwix (offline wikipedia/stackoverflow).
Dropbox?
You can also use Files to directly browse and access files from third party cloud services, although it requires you to download the service's app first[1].
[1] https://support.apple.com/guide/iphone/connect-external-devi...
But my use case has involved connecting to a remote host over a ZeroTier[1] VPN, using an iPhone 6S. I haven't experienced the scenario you mentioned; most problematic issue I've had is sporadic latency spikes during folder navigation. I haven't really done any true stress testing of things, but it's handled transitions between wifi and cellular network without any noticeable issues.
Looks like there are two different USB adapters Apple sells, and the product page for this[1] one mentions it can be used for quite a few USB devices, including "USB peripherals like hubs, Ethernet adapters, audio/MIDI interfaces, and card readers for CompactFlash, SD, microSD, and more."
Biggest issue for both external HDDs and other devices appears to be power delivery, which can be alleviated by using a drive with external power or leveraging a powered USB hub between the phone/tablet and the USB device.
[1] https://www.apple.com/shop/product/MK0W2AM/A/lightning-to-us...
Several apps (nPlayer, InFuse, LumaFusion) also support Samsung iXpand lightning/usb flash drives natively. With native app support, files can be played directly from the flash drive, without copying to the iPhone.
iOS storage management has improved with a user-visible "filesystem" and storage providers allowing edit-in-place, but there's still virtually no support for backup or rsync. The native iOS Files app is not a robust client for NAS storage. So far, the best option has been GoodReader (Russian devs) which implements robust sync (SMB, SFTP & more) within the app, along with optional in-app strong encryption that goes beyond iOS data protection. Unencrypted files are visible to other apps, https://www.goodreader.com/
Samsung's iXpand has built an ecosystem of iOS apps that support their custom protocol for iXpand flash drives via Lightning. Now that iPad enables access to local storage via USB-C, we need a similar ability to mount a ZFS drive, even if Apple won't provide this natively in iOS.
With a low-cost x86 SBC like Odroid H2+, an entry-level NAS can be constructed with Ubuntu ZFS and dual 3.5" drives.