iPhone Dropbox lets me edit txt files. I also had to reformat even though mine was ext4 but i was using ecryptfs which wasn’t supported.
For encryption, I'm using dm-crypt with LUKS on my root partition on Arch Linux: https://wiki.archlinux.org/index.php/Dm-crypt/Encrypting_an_...
It works pretty well. I didn't have to mess with the underlying encrypted LUKS partition/volume in order to reformat.