It turns out that you can turn off the soft keys if you have rooted your Android install. I used Termux, but I'm quite experienced with a unix commandline.
You need to edit /system/build.prop and find the line
qemu.hw.mainkeys=0
and change it to:
qemu.hw.mainkeys=1
The /system volume is mounted readonly so even as root you cannot simply write to that file. As root you can
mount -o rw,remount /system
to make it writable, but while it is writable, any process with write access can change it. You should change it back as quickly as possible after changing that file with:
mount -o ro,remount /system
For those who don't know what I just typed, you might find the following you tube video helpful:
https://youtu.be/ePz7BElFe8gAnd once you are done reboot, the buttons will be gone.