Tailscale will give you encrypted access to all devices everywhere, including iOS. For any hardware that can’t run Tailscale, you can use any Tailscale client on the same network to be a subnet router - other Tailscale clients can then access that network via that client. I do this with a Raspberry Pi.

Once you have a mesh network like Tailscale setup, you can use native tools to copy files, etc, because the the mesh network provides the connection.

Checkout Syncthing and Resilio Sync. Both are great sync tools with different features. I use both, but rely primarily on Syncthing since it’s much better on memory use on Android. I use Resilio just for its on-demand sync feature.

Syncthing can also run on an Rpi. I’m pretty sure Resilio can too.