I would be interested to see the playtests, as the odds appear to be with the proposed Defenders.
Consider this, a tricked out Han/Millennium Falcon plus an equally high PS Xeing with all the bells and whistles (in other words, your typical Han Shoots First list) will have 6 total Attack dice and 18 defense dice (Hull + Shields); if you go with a pair of Rookie X-wings, you get 9 Attack dice and 23 Defense dice! Compare this to a pair of TIE Defenders: 6 Attack dice (+ another "2 dice" if you factor in the reduction to target agility due to the effects of Tractor Beam upgrade) for an effective total of 8 Attack dice and 12-14 Defense dice (depending on whether we go with the 3 or 4 shield version of the Defender).
I think you're underestimating agility here. The YT-1300 cannot evade anything, and at range 1 can only evade 1/4 hits every turn. The big ship relies on its shields and hull enormously. Meanwhile, the YT-1300 firing back is going up against 3 defence dice + tough hitpoints. Modifications tip the balance even more to the Defender.
In any case, it's your custom card so you can do whatever you like!