There is absolutely no mineral that a TL9 culture could not more efficiently synthesize from space resources.
There's a difference between "obtain AT ALL" versus "obtain easily at reasonable/economic cost" when including the expense of transport. Sure, there might be iridium in those planetoids ... but you have to go find it and mine it and smelt and refine it and transport it to somewhere useful before the stuff has value. Each of those steps costs money/credits. How much each of those steps costs determines how economically viable "doing that here" instead of doing it somewhere else is going to be.
Yes, the resources might be "out there" but if you can't exploit them at a reasonable price, they're not worth the effort to go get them. Kind of like how if there was tons and tons of pure gold in Low Earth Orbit, it wouldn't be economically viable to send chemical rockets up to go get it and bring it back down to the planetary surface since it would cost more to go get the stuff than it would be worth on the ground (and if you significantly increase the supply, good old supply vs demand will lower the price you can sell it at). So even if there were trillions of dollars worth of pure gold orbiting Earth right now, it wouldn't be worth the effort of going up and "mining" the stuff to bring back down to the planetary surface. Why? Because it costs more to go get it than the value yield return after it has been gotten. You would literally be losing money on a business hauling pure gold down from orbit.
TL=9+ Traveller tech makes interplanetary resource exploitation a lot "cheaper" than it would have been at lower technologies, but it's still not going to be happening "for free" in the grand scheme of things. Just because lunar regolith exists doesn't ipso facto mean that concrete made using lunar regolith is going to be economically viable anywhere else in a solar system due to the added costs of transportation (even if transporting in bulk). Much like today's real world, distance equals time ... and as we all know already, time equals money ... and long supply chains are subject to disruption for all kinds of reasons, so security costs money too. So it all comes down to a question of how "easy" it is to obtain resources (with "free" being the optimal cost for things like life support on garden worlds).
The only reason to actually plant a colony would be if it is a garden world and people want to live there or exploit some biological resource.
Every colony is going to need to exploit some kind of natural resource, whether it be animal, vegetable, mineral or even just simple energy harvesting. Colonies only "grow" if there is an economic reason for them to exist in the first place ... and if you're setting up shop somewhere new, there needs to be something THERE for the colony to exploit in order to justify its existence (even at a subsistence level). The limits of how "large" a colony can grow will be determined by the number and quantity of resources it can exploit profitably. Hopefully, that limit is "high enough" for the colony to grow to a size that is self sustaining (Population: 7+, which is no longer Non-industrial) and capable of diversifying its economy beyond mere Resource (Curse) Extraction beholden to the interests that set up the colony in the first place.
You don't need to be a garden world for that to happen (although being a garden world helps tremendously!) ... but there does need to be something THERE that justifies the work and expense that goes into building a colony that won't be a permanent drain on the polity that sent the colonists to set up shop in the first place. Even if it takes centuries to realize, there has to be a Return On Investment in the process of colonization, otherwise the colony will "fail" for reasons various and sundry.
Think about the movie Avatar where the Sky People set up a colony on the moon Pandora ... despite the fact that the flora, fauna and even the atmosphere are hostile to humans (need filter masks to breathe the tainted atmosphere). What was that colony there for? Answer ... Unobtainium ... a stupid rock that sold for 20 million a kilo back on Earth that paid for the whole party on Pandora. In Traveller terms, that would be GCr20 per ton ... enough to finance a mining colony that needed a heavy security presence and at which survival was NOT guaranteed due to the hostility of the environment beyond the fence. The day 1 safety briefing for new recruits had the military commander tell them (to their faces!) that he would NOT SUCCEED at his mission to keep them all alive, because Pandora was just that hostile of an environment. The colony existed to exploit the natural resources by mining and ship the refined product out for profits. If the "unobtainium" wasn't as valuable as described, none of them would have been there, because Pandora wasn't the kind of place where anyone would want to set up shop for a colony of civilians who just wanted to "settle down peacefully and raise families" because Pandora was simply too hostile of a world environment to pull that off. When EVERYTHING around you is trying to kill you (including the atmosphere), just simply living until tomorrow becomes expensive.
Yes, you can build habitats on the sea floor at the bottom of the ocean with sufficiently advanced technology.
That doesn't answer the question of WHY anyone would want to do that.
