Don't know if you found the answer you were looking since this post ,as it has been 3 years now but these commands work. It was only the 1st part of your command line that was wrong
pm install -r --user 0 /
You can also change the " -r " to a number of different options, depending on what you want for the APK to do
ADB command cheat sheet
https://www.automatetheplanet.com/adb-cheat-sheet/
This command is if the APK is on device already in download folder on firestick, if in different folder change the path accordingly
pm install -r --user 0 /storage/emulated/0/Download/nameofapp.apk
Example
pm install -r --user 0 /storage/emulated/0/Download/TDUK-4k-Helper_1.3.apk
I've found this method to be fastest. I keep a folder of the best, and most up to date streaming apks that work on fire TV devices on my Android phone and use an application called Send files to TV (Which is on Google play store on my phone, and on the Amazon app store on any fire TV device. Then I send all the apks from my phone to the fire TV device using WiFi connection, which usually takes less than a minute with this apk. Then use remote ADB shell app on my phone to install all the apps on fire TV device using ADB command, which installs them without installation permission prompt. The same commands will work with a PC ADB command
Send files to TV
https://www.google.com/url?sa=t&source=web&rct=j&url=https://play.google.com/store/apps/details%3Fid%3Dcom.yablio.sendfilestotv%26hl%3Den&ved=2ahUKEwiHq6O5kP7pAhWyKH0KHar-AwcQjjgwAXoECAEQAQ&usg=AOvVaw1GU9VjAetrW_0uC3zJQy99&cshid=1592028553525
Remote ADB Shell
https://www.google.com/url?sa=t&source=web&rct=j&url=https://play.google.com/store/apps/details%3Fid%3Dcom.yablio.sendfilestotv%26hl%3Den&ved=2ahUKEwiHq6O5kP7pAhWyKH0KHar-AwcQjjgwAXoECAEQAQ&usg=AOvVaw1GU9VjAetrW_0uC3zJQy99&cshid=1592028553525
Make sure to rename APK file so there are no spaces ( you can substitute with the following " _ - . " or any of these characters or symbols that are commonly found in APK file names. " ( ) { } [ ] " Also if you have multiple files/commands to run you can add them to your script by putting a semicolon " ; " between each command and run them all at once on 1 ADB command script
Hope this helps