As long as it still boots, you can undo the change with a simple adb pm enable [packagename].
I wouldn’t recomment disableing system critical things like systemui. You can google each package together with “Can I disable X” and you should get decent infos.
Regarding the launcher:
I don’t have a Fire TV but I had a Fire Tablet. So if these two work the same, you can install another launcher without issues. But Amazon removed the setting for default launcher, so it will always pick the stock launcher when you press the home button.
To override that, there are two options.
Install the Automate app by Llamalab and make a small flow that detects when the stock launcher is the currently active app and then automatically launches the new launcher. This option is completely safe.
Disable the stock launcher. If Android doesn’t find it’s set default launcher, it will instead open the first launcher it finds. Worked good on the Fire Tablet, but I can’t verify that this doesn’t cause issues on a Fire TV. So this might be a bit risky.