but I don't understand why my escort takes all the fire while there are two other beam boats not even being touched.
The AI attacks whoever they perceive as the biggest threat.
One strategy you could do, is to fly outside of the firing range of the cubes and tactical cubes (10km +). Chances are, it's going to chase after you (slowly) and not fire on anyone. This is pretty much free hits for the rest of your team. But, you need to keep watch as to who the Borg is targeting (you see the Borg information at the top, to the lower right of it shows you who it's attacking). Once the target is changed, fly in and attack again until it changes target back to you.