Requirements:
Laptop: openssh, a file manager
Phone: Termux or an SSH server app, a file manager capable of connecting to a file server
Prerequisite: Both devices should be connected to the same network

I do not use emails on my FreeBSD machine so I did not bother setting it up. But it runs at startup and apparently prevents other services from starting till it finishes its job (I cannot ssh into this machine till sendmail decides it has had enough and stops).

Using a DE is pretty straightforward but you might be out of luck if you use something like dwm.

Using normal methods to copy sparse files will only take up so much time and actual storage. Pass -S to rsync to tell you are transferring sparse files and it take care of it.

Boot with live media and chroot into existing install. Install and enable dhcpcd.