TrueHD has only ever required HDMI 1.1 (since 2005)

Software limitation, not hardware.

You can replace the UI on the Shield if you want via sideloading, mine essentially looks like an Apple TV

Debloating just disables a few random background processes, installing the play store doesn’t really do anything because you can sideload apps anyway. Both of these things only run because Amazon allows them to, they could disable sideloading without making a whole new OS if they wanted.

This isn’t possible. It has a locked bootloader.