You can use mixplorer (6$ google play store) to quickly launch a local ftp or smb server. attach a hdd or ssd to the charging port, buy a dongle (<25$) that allows data transfer, charging, and ethernet. And you basically just made a nas. You could even port forward the ftp to make it accessible outside your network (i dont recommend this unless you are hosting a vpn).
do not expect good transfer speeds but as far as a persistant redundant secure storage this is a great solution. The phone used would optimally have usb c for better data tranfer speeds. But the phone itself may limit these speeds refer to your phones specifications data for that (just google it).
The phone might overheat so maybe slap a usb fan on it and remove the case. If possible make it "float"(put rubber feet on the phone or smthn idk) to allow airflow on both surfaces of the device.
Very cool Idea. Please make an update post and link it in this thread if you make it and test speeds <3
Edit no idea how i forgot to mention this, but ADB can also be used to send commands directly to the device over the network you have alot of fine tune control over the device, you can run, close, install and uninstall apps, run scripts, remove default unessary bloat and even the launcher! After setup you may never have to touch the phone again, barring power loss etc. If that seems like alot there are also methods of remote desktopping into the phone. But in my expereince those solutions are laggy and slow. But if you just need it for simple tasks this might be optimal for you.