At 12m they NEED to have facilities on a barge waterway or at the test/launch site, while at 6m trucking, barging, or flying (in a Super Guppy) a upper stage are all options. I didn't say they were easy, but much more so than a 12m beast. It doesn't have to be as quick as moving a F9 because they would only need 2 or 3 per year, with the stages flying back to CCAFS or Vandy after missions.
It has nothing to do with being quick. Either a road (with overpasses and powerlines) can handle a 6m diameter payload, or it can't. And I'm suggesting that trucking a 6m diameter payload from Hawthorne to McGregor and the launch sites might actually be impossible.
Yeah, I don't think trucking cross-country like F9 would work well, but a combination of trucking, shipping, and maybe flying should work with existing assets.
I'm thinking more like trucking from Hawthorne to the Marina del Rey (Endeavor
and ET-94 did a similar trip), then cargo ship to one of the Texas ports, truck inland to McGregor and back, then cargo ship to Port Canaveral. Or lease a Super Guppy or Beluga from LAX to McGregor to Canaveral.
It's not impossible to move a 6 by 25 meter, 20 tonne stage and utilize SpaceX's existing sites. It's quite impossible with a 12m stage, especially the ITS booster.