From my experience the Z8350 should be capable of decoding 1-2 1080p60 through VAAPI and rendering with OpenGL. Have you tried using mpv from the command line w

From my experience the Z8350 should be capable of decoding 1-2 1080p60 through VAAPI and rendering with OpenGL. Have you tried using mpv from the command line without XOrg running? i.e. something like: mpv --hwdec=vaapi --opengl-backend=drm --drm-connector=1.HDMI-A-4 ...