Although the systems in TU are not exactly analogs (after all any planetary system will contain virtually unlimited resources) to countries here on earth, I think it can be instructive to compare them anyway.

Consider the cases of Japan, India, & China.

All three began a process of transformation after contact with the West (similar to First Contact in the 3I, esp in the early years) which has resulted in increased industrialization albeit at very different rates & also w/ fluctuations in those rates over the years.

I believe that India & China were Pop8 & Japan was Pop7 when they were first 'opened up' to the West.

In that period of time look how they've changed.

Now there are also lower pop countries here on Earth that haven't really changed that much while some have changed quite a lot but all possess enclaves where one can find the highest tech avail while still there are also many areas where it's very, very primitive.

I wonder if the Pop level is the most significant factor?

If the Pop is high enough, like Japan, China, & India, it almost seems as if there's a sort snowball effect that compels (propels?) a country into a development cycle that may be resisted, & or suppressed but nonetheless progresses towards higher tech.
BTW, are there any other countries that reached Pop7 before they were 'opened up' to the West?

Sometimes there also seems to be a similar effect on very low Pop countries but not always & most of those (in the Arab World) are due to fortuitous natural resources although Syria , Lebanon, & Jordan seem to be unusual cases in that they seem to be retrogressing somewhat (well, maybe not Jordan so much) after advancing w/o a whole lot of natural resources. (Yeah, I know Syria has some oil but I don't believe output was ever all that much).

In any case, looking at countries here on Earth it seems that development/advancement can be all over the place so Im guessing that that was a big influence when the TU was first conceived. And, it did look cool too!

Could it be that the new ultra-low trade levels that T5 seems to imply is intended to insure that systems remain insular so that homogenization doesn't occur?

Could the 3I or other states, large & small, exist in a TU like that?

Would an urge to expand/conquer (Alexander the Great, Genghis Khan, Manifest Destiny, etc) be enough to initiate, sustain, & then maintain something like that?
(After all when the Terran Confed took over the First Imperium, I don't really see that the TConfed gained a whole lot. Now the Fleet & many individuals/families did gain a whole lot, esp that Admiral who gained a new Imperium!

Now, of course we know that the Vargr & Aslan have their imperatives that drive them on so could we say that about humaniti too?

Is the 'driven by commerce' concept NOT the real reason but rather a 'driven by personal/family/corporate' acquisitiveness  (which may not be limited to financial profit) concept that really drove & sustains the 3I?