If you're serious about wanting to deal with plaintext on Android, I recommend installing Termux and learning how to use the Nano text editor (via Termux). Here are some links to help you get started there. Also, I recommend getting a bluetooth keyboard.

* https://f-droid.org/en/packages/com.termux/ (download Termux; it's also on Google Play, but the latest version is here; scroll and click the link that says 'Download apk'--not the big button that says 'DOWNLOAD F-DROID', unless you want to install F-Droid and download it through there, which is said to be more secure; if you've never installed an .apk file before, you'll probably need to change an Android setting that lets you do it)
* https://www.growspice.com/Termux-tricks-td3491.html (some basic shortcuts that are useful when using Termux, to help you multitask and such)
* https://www.growspice.com/A-desktop-user-s-guide-to-Nano-a-command-line-text-editor-td3471.html (How to use the nano text editor.)

Okay, so once you have Termux installed, you basically have a Linux command-line app; no GUI, but you don't really need one terribly. You can install and use all sorts of programs that make your tablet useful--and you can make your own, if you're a programmer. As an added bonus, both Termux and Nano make it easy to multi-task once you learn how (in Termux ctrl+alt+c makes a new terminal; ctrl+alt+up/down moves between them; in Nano, you can move between open text files with alt+left/right).

So, in Linux, to create a text file, you can do it plenty of ways:
1. `touch myFile.txt` (that will create it if it's not there)
2. Type `nano myFile.txt` edit it, and save it.

Stuff like that. Learning how to use the Linux command-line will be important if you take this approach. You'll want to know commands like this:

* `ls` to show the contents of the current working directory
* `cd myDirectory` to change the directory to myDirectory
* Follow just about any command with `--help` (e.g. `nano --help`) to learn more about how to use it. `man whatEverMyCommandIs` also helps (it opens the manpage for it, if available).
* `rm myfile` to delete a file
* `rm -r myDirectory` to delete a directory/folder
* `mkdir myDirectory` to make a new directory
* `chmod +x myFile` to make a file executable
* `./myExecutableFile` to run an executable file that is in the current directory.
* `exit` to exit the shell (note you can type `bash` to create a new one running inside of the current one, so if you exit, you'll return to the old one)
* `pkg install myApp` to install a program called myApp (if it exists in the package manager); I don't remember if Nano comes pre-installed; if it doesn't, type `pkg install nano`.
* ctrl+c and ctrl+z are useful if you need to force quit a running program.
* Press tab to autocomplete stuff you're typing (like file and directory names).
* If you can't find a program for your purpose, see if someone has made one with Python; Python works on Termux, and so does pip (so, you've got access to lots of stuff).

There are more Termux-related apps on F-Droid, if you want to be able to do even more stuff with that.