The only way to improve the distance is to get the antennas higher at either or both ends.
Or use a vertical beam.
The advantage of a directional antenna becomes evident when you have multipath distortion. Many years ago, I was to install a FM BC receiver + antenna for background music. I used my portable "field strength" receiver and a 3-el yagi on the roof to determine optimal antenna location and aiming. As I aimed the yagi at our 35KW ERP transmitter (LOS about 30 miles away), the signal strength was good but, the recovered audio was badly distorted.
The receiver site had a STEEP hill immediately behind it. As I turned the test antenna, the signal came up higher and the distortion went away when I aimed directly toward the steep hill. That's not the first time I wound-up aiming in a different direction. Had I had a vertical (or other omnidirectional antenna), I never would've had an acceptable result.
All that said, why not ALSO buy/build a portable 2m yagi or quad? units such as the MFJ-1763
may be workable.