A commercial/trade arcology is possible as I mentioned, Planetary Diversity - More Arcologies already made it. It's just mabye the secondary economic center idea could be hard to implement.
The trade collection is the difficulty I was referring to, yes.
Trade arcology is interesting, but has a separate issue in practice, which is the question of Ecu self-sufficiency. The design concept of an Ecumenopolis system is pretty clearly one where an empire is rewarded with extra efficiencies for building the entire empire around around providing resources for the Ecu to refine, with the catch that if the resources stop (say losing the colonial frontier in a war/robot rebellion) the ecu starves of food and minerals. First League origin and all that. This is in contrast to the Ringworlds which are the self-contained autarky economy package, where as long as you have a base level of minerals (say the Matter Decompressor unlocked by the same ascension), Ringworlds can cover all their own needs.
A Trade ecu runs afoul of that, because- once built- it basically becomes functionally self-sufficient. You wouldn't be building a trade ecu without a trade build, which means a trade federation (bonuses that maketrade economically viable), the trade federation policy (turning trade into both the energy
and consumer goods needed for upkeep), and Mercantile (increasing the trade numbers and Merchant jobs from the presumed trade-district). With all your building slots unlocked, you can build hydroponics bays to cover the food and the refineries for the strate resources, with your only really necessary input being the mineral input for the refineries... which could well be bought with the trade.
Now, an optimization player wouldn't
do that because it'd be more optimal to spam trade buildings for more merchant jobs and keep the empire structure, but the fact that you
could do that illustrates the balance issue, since- even if the trade-ecu did lost their resource worlds- they could use their money to buy the minerals to change into something more self-sufficient.
Which an Ecu shouldn't be.
I think a strategic resource arcology is good because strategic resource buildings are so inefficient and if you want to spam habitats, it will cost your performance.
Arcology buildings are
already covering for the strategic resource efficiency. The pops and energy/admin upkeep you are saving by moving former industrial worlds into the arcologies can be put to work on refineries for net strategic resource surplus. As you de-populate previously established planets, converting them into refinery worlds is a good way to make use of them as part of your overall 'resource worlds fuel your arcology.' Heck, you can put the refineries
on the the Ecumenopoli already- they max the building slots, and benefit from the Ecu 20% bonus. That these take basic resources doesn't mean they're in-efficient.
If you need more basic resources to fuel your Ecu, the solution is a better balance of resource worlds not to make the Ecumenopolis require less resources or produce their own needs. Not-being self-sufficient is half the point of an Ecumenopoli build vis-a-vis a Ringworld build.