Have you tried (& succeeded) hiding the onscreen buttons and using gestures instead
I tried getting rid of the nav bar and used the pie controls, but it just feels weird , it's not instantaneous like the on screen buttons. I think gestures would be a better option, something like swipe left back, swipe right :multitasking swipe down:home
I know there's a "swipe back" mod on Xposed, but it doesn't work all that well, with a lot of apps it just sends to the homescreen and sometimes the previous screen becomes the "wallpaper" when I'm on the homescreen.
I tried too an app called "swipe home back" but it just some presets and I cannot set the way the gestures work