It's also possible to run sshd in Termux (https://wiki.termux.com/wiki/Remote_Access), and mount a regular sshfs on the Linux side. Though this may not allow modifying files in external storage. The Linux side being:

    sshfs -o follow_symlinks,IdentityFile=/your/termux/id_rsa -p 8022 Linux@$PHONE_IP:/ /path/to/where/to/mount