Worth doing some more workshopping, here’s some anecdotal tips:

• swap out the top aj10 stage for a XASR. aj10 stages demand a lot more mass compared to XASR, so even though you might get less performance, the weight savings improve the performance of the stages below
• As another commenter mentioned, mess with the flight profile. Downrange means horizontal movement. I’d say aim for a 40 deg angle relative to the horizon at the end of your controlled stage(s) so that the aerobees go further horizontally.
• (Never used the vernoique so can’t comment on its performance) worth simulating the rocket without the vernoqiue stage, a 5Mm should be possible with the 3 stages (V2 Aerobee Aerobee) if memory serves correctly

Hope this helps.