¿qué debo mejorar?

Agradecería de sus consejos para poder mejorar el enlace de 8km sobretodo en el tema de la latencia.

With those numbers, latency will be a funciton of your SNR not your signal strength (which is what you have shown). To get better SNR, you can use larger antennas which will focus the signal to a narrower beam or you can use a different polarization scheme. You are currently using linear polarization, but rotate the antennas 45 degrees to the side and you will be using slant 45. Be sure to ensure the same polarity by rotating one antenna left and the other right or you will have a cross pol link, which may still work.
You can also look at specially separated antennas (two antennas per side about 15ft apart vertically) to gain SNR. Or place the antennas higher up the towers.

You can also use shorter links but at 8km you should be able to just use one unless there is a lot of trees or buildings in the way.