To make it clear, the "thick shell" of the Zodiark is of course not just the 10 layers armor, which the others have too, and it isn't all that dramatic to begin with. But since the shell is explaining how the design can be shielded against space erosion so good, and the main reason why they are so good encapsuled are actually the 24% engineering spaces, I count those to some degree towards the protective shroud. That would make it around 8 times as thick at highest, but I assume around factor 5 - whatever, it is a made up mind explanation.
Also, the reason why building all the ships took so long were not the shipyard times, but resource shortages. Everyone is 260kt duranium, and then also around 73kt mercassium and 103kt boronide. Rest irrelevant. To use the time, I built most components ahead already like the cheap reactors or the rather expensive cryogenic berths. The real assembly thereafter was ridiculously short. Despite only having one slipway, I managed to manufacture all the 14 super-freighters for example from 5.2308 to 4.2312, so around 4 years, meaning about 4 months build time per ship. That is nothing. After those I was again a little short on minerals yet again - mainly because the asteroid miner would consume my rarest and most needed resource, corundium, in amounts that would equal a couple thousand auto-mines.
He however holds by itself more mining power than the whole rest with its 6 dedicated mining ships combined.