We have a Shield Pro and unfortunately flashing wasn’t an option of us. What we did was start using an “alternative” launcher called Projectivy and installing apps outside the Play store.
Casting isn’t possible too, we simply learned to live without it. It seems you can use it on Kodi but I never really tried.
I remember the main dev of Mull saying it can be done on about:config but you need to disable something else, maybe you can find it on the project repo