Hmm... tank immune to instakill and shell ammo or lasers.... now THAT would be a tank... I see the tank as slow as hell (around 16 or so would make it half as fast as a bomber), with more armor and firepower than a bomber, about the same reload time (or slower), but a shorter range. It would hit hard and slow. Tanks would take a while to get into place during an assualt, but be devestating once they were.
Basically a slower more heavily armored version of a bomber (kinda like mini siege starships). Bombers would be able to take out tanks, but tanks would be almost useless against bombers and fighters. Fighters wouldnt do much to tanks, but swarm ships would be deadly. Tanks would have the same bonuses as bombers pretty much (heavy, ultra heavy, structural, ect).
My brain is fried because its 4:50am and I'm still in the middle of writing a paper, but right now this sounds somewhat logical... though it might not when I wake up and read it