1.) Download & install LMT launcher HERE. This allows you to use the PIE on-screen menu navigation softkeys. I love it even moreso than the stock Android softkeys. Here's what my 8.4 looks like with LMT enabled:
(The PIE navigation only show up if you swipe your finger from the side of the screen)
2.) Configure LMT as you deem necessary. It works right off the bat, but you'll probably want to mess around with the settings to your taste preference.
Now to disabling your capacitive buttons:
3.) Download a file explorer/manager (I used Root Explorer)
4.) Using Root Explorer, navigate to System > usr > keylayout, and open Generic.kl in Text Editor. Say "Yes" when asked if you'd like to remount the file as read-write.
5.) Once the file has been opened, navigate to key 158 (back) and key 254 (app_switch), and put a "#" sign in front of each line. Save the file. After rebooting, your back and appswitch capacitive keys will no longer function. I personally left the home button enabled, but you can find the home button key in that same Generic.kl file and disable it if that's what you'd like to do.
6.) Next up is disabling the backlight on your capacitive keys, which was waayyy easier than I ever thought it would be. Simply navigate to Settings > Device > Display > Touch key light duration, and switch the setting to "Always off". Wah lah! Now your capacitive keys are disabled and their backlights no longer light up.
That's it! You are now all set to go with disabled capacitive buttons/backlights and have a handy on-screen PIE navigation menu. No more accidentally bumping the back button in the middle of a game. No more distracting backlights annoying you when you're trying to watch Netflix.
There's also a way to add the stock android style soft-menu buttons, but I was only able to make it work in portrait mode, not landscape. I prefer LMT anyways. However, if anyone wants to add the stock android soft-menu buttons, let me know. I can always add it to this guide.