It is important to manage expectations. While x265 encoding is impressive, a 300MB file will inevitably suffer from:
Typically downscaled to 720p (1280x544 or 1280x720) or even 480p to preserve sharpness at low bitrates. Framerate: Keep it "Same as source" or set to 23.976 fps. Use "Target Size" mode and set it to roughly 300MB Movies