In general better antennas, better antenna placement and height of antennas are going to gain you more distance than cranking the power, although that helps too, but optimizing the antenna will get you a long way on much lower power.
If you are generally working in the same direction from the base then a directional antenna (yagi) is a huge help, of course if you go off the backside with your receiver (rover) it will be far worse than an omni-directional antenna.
Longer wavelength (lower frequency) radio waves propagate further too, BUT generally for RTK we are going to be stuck in the UHF spectrum (460 Mhz or so) so can't change that much.
I have made voice communications on amateur radio frequencies (7-18 Mhz) with a few watts output (under 15) with a wire antenna at 1000's of miles, but those were in MUCH lower frequencies than what we have available for commercial data transmissions.
SHG