Not sure why the link doesn’t work: https://lemmy.ml/c/av1@lemmy.ml (there is also a second community and a av1 discord server.

Fire stick pro max should support av1 but I have no experience with that.

Av1 is the best compression format but it still requires more work to use it well. The tools for x265 are more mature.

Alternatively you could look into av1-svt and av1an (!av1@lemmy.ml). But there are far more tools for x265