This is what I’m using. It’s kinda ugly but totally ad-free and does what I want but not one bit more. I installed flauncher, connected to the shield with network adb, disabled the leanback launcher, then hit the home button on the remote. Easy peasy, and much better than remapping buttons IMO
In the current version it was already advertising / recommending shows and movies too me I have no interest in. I just want my plex content and that's it. A custom launcher is great for that.
Same here, one other improvement I would recommend is remapping the Netflix button to something less annoying/more useful in case you press it accidentally. If you have Hue lights, you could make it toggle your lights with this app for example.