I'm curious what your plan is for the players getting around in space. Are starships going to be available for free (or on lease, or something) to groups of starting players? That was one of the reasons I had suggested a one-system exploration area, the ships required to shuttle around are a lot cheaper, and won't require many specialized skills to use. They'd also be less capable of long-duration support for an exploration team, which leads me in to my next question.
Great questions and guidance - thank you! So, to your point, shuttles will or can be available to starting PCs. Mainly Bossman, Vosh and Skylarks. I'd imagine they'd be available primarily on leases or patron-based loan/mission use, but depending on PCs, someone(s) could own one.
The primary base of operations is the Kiwi-Washman system. With two habitable planets, the space station, and untapped asteroid belt and gas giants, there's lots of possibilities, as you've noted.
Are there going to be civilized worlds in the area we're exploring? A big part of the West Marches setting is that outside the one town, the rest of the area is wilderness with no civilization. Some character concepts and careers may not make much sense if there is no civilization to play in, but having other towns makes it hard for the west marches style to work as players can go out and adventure indefinitely, never coming back to the starting base.
My thinking is that there are no known worlds, and no allied / safe / settled worlds in beyond Kiwi-Washman. There may or may not be first contact potentials out there, but nothing initially like a civilization. All supplies are brought in via Coreward traffic, or cultivated locally (Kiwi-Washman, or systems just Coreward of it).
As a conceit, if PCs head back Coreward, they're effectively out-of-play for the current game cycle. That is, they can be pursuing downtime / off-camera things, likely which require more time (training, etc.). Not required to go Coreward, I think, but if folks do, they're leaving the sandbox basically.
Some systems may open up, depending on Player and PC interest. Colony building is tough work ...