Full impulse is one quarter light speed. One light-second, by wikipedia, is 299 792 458 meters, so one light-minute is a little over 17 million kilometers, and a quarter of that is still over 4 million kilometers. The radius of Earth is 6,378 km, so the diameter is twice that, but you're still off by three orders of magnitude. (And since each ship is less than one cubic kilometer, you also have no justification for hundreds of ships not being able to fit. Hell, you could fit a fleet of hundreds just in Earth orbit so far away they could barely see each other.)
A ship traveling at full impulse, as any ship entering possible enemy territory would obviously be, would travel the diameter of earth in less than two TENTHS OF A SECOND.
If a Borg cube can traverse the entire effective range of a Thalaron blast in that amount of time, it's just intuitively obvious that each ship would only be able to take out one; they would obviously have to track and follow one particular Borg cube.
Even IF the Thalaron blast could be sustained over the exit portal, and take out a number of Borg cubes rapidly, the Borg have instantaneous communication and would be able to get the cubes behind to target and attack the vessels creating the Thalaron blast.
Again, space is HUGE. Even one quarter of one light minute is a vast area of space. There's no possible way to take out 7,000 cubes in 64 QUINTILLION cubic kilometers of space with a few hundred ships that can be destroyed in a matter of seconds. Period. You're completely delusional if you think otherwise.