Good range and video quality is mainly created with good antennas and not by the transmitter power. To much power is not what we want as it will interfere with the other onboard equipment like servo’s & RC RX. As I use a rather high video frequency circular polarization is the way to go. There are several circular polarized antennas for sale. And many tutorials online to DIY. Below a very good and educating video.
I use Spironet antennas, 3T & 4T Helicals & a 5DBi CP patch.