More like the Borderlands Please

[Vormaerin]

But The Traveller Adventure's whole point is that even a J-1 fat trader can just fit demountable or better collapsible fuel tanks and multi-jump through empty hexes.

(Plus quite early on they get to hijack an actual far trader which makes the March Harrier redundant...)

IMTU empty hex jumps add significantly to the astrogation difficulty and thus chance of misjump and are therefore illegal for any imperial ship carrying freight or paying passengers - and more importantly ships under mortgage or subsidised contracts are generally forbidden by contract to undertake them other than in emergency circumstances because until it is paid off your ship belongs to the bank or subsidising consortium and not you!

Travellers being Travellers they will of course ignore this but at their peril.

Which makes mains and clusters meaningful again

yes, there are ways to get around gaps, but that doesn't have anything to do with the point of the discussion, which is that if the players have a J1 ship, they are staying in a confined area. Which allows a GM to develop a small area in depth instead of needing a huge area (that will inevitably be shallower).

Yes, they can change the ship so that it isn't J1 anymore, but that's not the point? That's the thing you do when the group has outgrown the starting area. If you ever do.