3. I consider Major Cities to be significantly and notably more vulnerable than Small Hives, due to population capacity and increased defenses from the latter.
The other stuff is debatable, but here I think you're looking at it the wrong way. In Tarascon we will have a single city which is too distant to receive any reinforcements in a timely fashion should the planet be attacked. If someone wanted to seize as many possible examples of our technology, their best bet would be to attain orbital dominance (which they need to do to land anyways), and send all their troops to attack the Lonely Mountain, get in, steal everything they can and then leave. The city will have only what troops we initially send there, and no city is close enough to send troops to it to bail it out. Or alternatively, if Orks wanted to establish an actual base on the planet and seriously dig in? That's an ideal location for them to do so, with lots of high end tech to loot. Our Major Cities do not have these problems. Reinforcements for them are available by rail, air, and even ground if needed.
Further, there's good political reason to develop Tarascon. We seem to be leaning towards having Syr marry an Asgardian Knight, and the possibility of founding a branch Knight House and giving them a region to rule so that we can field our own Knights is enticing. Even more so now that we can make Psi-Knights. And a region with a theme around dragons is perfect for knights.