The BGS causes population to trade with itself
The thing about this is, I don’t think that “weak link” economies are actually a thing in the BGS/markets of the pre-colonization/NPC systems. Take an Agriculture/Refinery system for instance (I have been using one as a supply station for colonization with my carrier). There can be several refinery economy stations throughout the system, with usually refinery hubs on the same planet as the station orbiting or on the planet… but the agricultural port orbiting a water or Earth-like world in that same system has zero refinery products and those are generally not consumed by agriculture anyway.
There can even be colony or military ports on those same planets (on the surface, I mean by that) without much of an effect from those hubs. So the colonized system economy rules have nothing to really do with how old systems or their BGS rules work.
Now… is it realistic to a certain sense that you would supply in-system than to go look outside of it? Probably, yes. But this is a bit ‘meh’ to most players who are rather building security stations and similar military infrastructure for their security effect rather than the military economy. Or agri for standard of living in systems where it’s otherwise low/risking to be negative, etc.
And maybe that’s meant to introduce some additional thinking into planning, fine by me, but the best (and possibly only) way to do it is just “Brute force it with raw influence numbers on a sufficiently large planet, keep interfering weak links to a minimum, and hope it works out to keep supply of everything - or mostly everything - intact”. Not exactly that clever if you really think about it, just encourages more leaving behind/abandoning of potentially decent systems with many smaller 1-2 slot bodies.
I also still hold on my position that it is less the weak links and more the forced planetary economy which
does not turn off when you do eventually specialize an economy into something. It should serve as a baseline that you
can take a station into but by no means be a hard-locked influence that can by no means be toggled off, even if you place refinery hubs on a high metal content world with volcanic activity (or space farms around a water world, which will not toggle off the tourism it also provides to ports). And they are set at such a strong baseline level, even muting them out with a brute force approach can only go so far.
What this results in instead is… people will just seek out other places to build in instead. At this point for refinery builds, anywhere with plenty of rocky bodies that lack geological signals. And you can only really specialize an ice world into industrial (or a hybrid industrial/extraction) if you wanted to do some light roleplay around pulling out whatever is locked up in their frozen crusts, because it’s always going to provide that baseline industrial influence and at an extremely strong level. And extraction is not an economy which gets a port with a baseline except for the asteroid base, which has its own placement restrictions (for obvious reasons).
Why I suspect the planet economies can’t be “switched off” (so to speak, rather than any hitting switches by the ‘architect’) is because they were implemented as a form of strong link behind the scenes but with high base characteristics, and so they’re also boosted by whatever boosts the economy types they are… but that’s another subject.