[Android Head-Units] aeonfluxcapacitor:
Thanks for the suggestion [MENTION=322374]joshnat[/MENTION], I was starting to head down that route (automating a FTP Server/Client approach) before I found what seems to be the exact solution I was after. There are definitely a ton of apps that can manage local WiFi syncing but hardly any of them are suited for my purpose.

In case anyone else finds themselves trying to do this I'll add my notes here. Most file syncing apps are geared towards manual and occasional device-to-device transfers. While some of these could be automated it would probably prove cumbersome and unideal for my use case.

Option 1 - WiFi File Sender

Pros:

Simple UI should be conducive to automation and otherwise lightweight app

Auto-discovery of devices running the same app on the local network (works when HU is connected over Phone, both devices find each other)

Good speeds between Phone and HU over Hotspot WiFi, all traffic sent locally.

Receiving device can define the path to receive the files/folder (essential if you want to fill a specific app data folder)

Multiple distinct files and folders paths can be selected to send

Any files already in the defined recevier's path will stay there during a sync (e.g. no rsync like behavior where files are removed if they aren't found on the sender side)

Cons:

No permanent settings, any paths you define are reset each time you load the app.

Only one folder can be defined to receive which negates some of the value of selecting distinct folders on the sender side. You could still automate the moving of those files on the receiver's end to their correct destination after the transfer completes but thats more painful automation.

No scheduling or automatic syncing upon device discovery

No understanding as to whether files are overwritten every time if the receiving path already has them or if existing files are simply skipped (didn't test this far).

For my case overwriting existing files just means the transfer will take longer especially when we're talking about a folders with GBs of data and automating this sync regularly to insure new files are sent over.

As this was one of the few apps that gave me what I wanted I began work on a Tasker profile with AutoInput actions but being new to AutoInput I was having little success in getting it do what I wanted.

Option 2 - Syncthing :victory: :laugh:

Apparently an open source alternative to what was BitTorrent Sync and became Resilio Sync. Which I had tried earlier (and why I had initially shrugged off Syncthing), finding it would not link my devices successfully making it a non-starter.

It turns out Syncthing is nearly the perfect solution for my needs. It seems very similar to an rsync style solution. I'm still testing it out but so far so good.

Pros

Automatic syncing of folders that are watched for changes (no job scheduling, changes are detected and syncing begins)

A folder can be defined as the master on a given device. If you only want the files on the phone to move to the HU and not vice versa this is the option for that.

Define individual folders to sync between devices. In my case DSub keeps its album art under /data and its music under /media so this is helpful

Can restrict devices to local IPs and turn off the use of relay servers.

Automatic discovery works but isn't necessary once you've defined each device on their counterpart.

One time setup of folders and devices as settings are presistent

Ability to limit traffic to local network and to specific WiFi networks (by SSID) I've had a little trouble with this so far so not sure it works as expected yet

Cons

File transfers seems a little slower than with WiFi File Sender but more testing is needed to be sure. See speeds of 1-2MB/s but sometimes dipping to 600KB/s. From reading other posts about Syncthing it sounds like the files are encrypted/hashed and otherwise processed and that can cause some of the slow down.

Files deleted on the master will be deleted at slave folders. I believe this can be overridden but it may result in regular manual interventions where your master folder says it is not up to date. Still need to test around this. Ultimately I may let them keep each other up to date since I will still stream the occasional uncached song from my HU.

With the exception of my HeadUnit killing most apps when it goes to sleep, thus requiring I start Syncthing on some condition so that its always running when needed, this app appears to be the solution for my use case with minimal automation.

Will report back if I change or improve this setup or find other issues. For now I'll build a Tasker profile to start this app when my hotspot connection is detected.