I wonder if you could dedicate some of the probes to serving as a relay network stretching all the way back by only partially accelerating them so the bulk of the probes will go as fast as possible and then a tiny fraction will go a little slower and another tiny fraction slightly slower than those and so on so that they spread out into a chain as they go. Of course, you'd only need to do it that way if you had to launch them all within a short window. If you were to continuously launch probes then you could maintain an evenly spaced chain for as long as you could keep launching them.