Are there any ways to install a third party keyboard as the system keyboard?
Way1: famous method now a days.


Unroot
Install Magisk (a systemless root solution) link
Install App Systemizer module for Magisk from magisk app
Make your app system app


This way you can make the user-installed app a system app without actually modifying the /system partition.

Way2: 

Change system partition evergreen method
Follow this guide link