Termux is great! I use it to ssh to my work or home systems for
maintenance when necessary.

From what I remember, it is possible to access external storage from
Termux. https://wiki.termux.com/wiki/Internal_and_external_storage

Personally, I use OPNsense at home and I have my phone set up with a
Wireguard connection to my home network when I am not connected to the
WiFi. Of course I also have things set up for remote access with no
CGnat and a static IP.

I use Automation to shut off my WiFi when I leave my home. Installed
from F-Droid. https://server47.de/automation/

WG Tunnel automatically connects to my Wireguard tunnel when the WiFi is
shut off. https://wgtunnel.com/

I also use File Manager+ which is from the Play Store, but it works so
well for SSH based file transfer and I haven't found a file manager app
in F-Droid that works as well.
https://play.google.com/store/apps/details?id=com.alphainventor.filemanager

If anyone has found a file manager app that works well and is found
through F-Droid, I would appreciate a link.

The reason I like my setup is because there's no duplicating of files
like Syncthing does. It's closer to a cloud storage setup, aka my
personal cloud storage.

Bracham