TL Expansion, or lack of...

[strontiumdog]

The answer is already all over our own world, and we don't even have interstellar distances and costs as barriers.

High tech spreads quickly through those nations able to support it, offer items in return that are of similar value, and those nations that cannot at least buy the tech, do not have it, or have small low cost HT items of personal use.

So we have third world nations with oxen as motive power for farming while jets fly overhead moving their upper crust out or tourists in.

So don't think of TL as isolated Red Zone contact situations, more like a measurement of first world/second world/third world economic states that can be supported by the productive activities and culture of the local economy, which shows what tech can be expected to be encountered commonly versus the occasional offworlder or noble import.

This. It's a common mistake of techy people to confuse technical methods and knowledge (which is easily transmitted, assuming you have an attentive audience) and capital accumulation. Only the latter makes you richer, the former is useless without the latter. Think of how many things we 'could' build if we magically had the right tools - nanomachines, space elevators, etc. - the core engineering is there. It's the money that's missing, i.e. the accumulation and coordination of production that's involved in a real-world economy. Likewise, societies which squander capital always fall behind technically, no matter how hard they try (i.e. the USSR).