Ive been trying to work around issues similar to this myself.
Ive already noticed screen burn on my pixel, due to the (almost constantly) black navigation bar.
Originally i used oreo tiles in the app store (as mentioned above)to enable immersive mode, but had horrible response time when trying to wake my phone up (common problem).
Then i tried enabling immersive through terminal emulator in the app store by typing "su" and then "settings put global policy_control immersive.navigation=apps,-com.google.android.googlequicksearchbox" and for the most part it worked, but i would often get stuck in apps. i would swipe up to press the back button and instead of sending the back action the nav bar would immediately hide again. this would leave me to swipe up and trying to double tap back super quick before it went hidden.
For the third attempt i used the twrp Oreo aroma mod installer in the themes and apps section to completely disable the nav bar and attempted to use LMT for navigation, but LMT would occasionally crash and not allow me to return to the app drawer to restart the service. So that was a failure.
I ended up using "nav bar apps" to simply recolor the nav bar based on the application that is open, so instead of stationary black, it changes colors.
Just sharing my experiences, maybe youll catch something out of it that will help you.