I asked ChatGPT this query:

"What countries form an economic ecosystem that is largely independent of the U.S., if any? A requirement is that a U.S. citizen can invest in them, that they be politically stable, and financially healthy. Perhaps Nordic?"

perplexity.ai - What countries form an economi

Summary: Yep, Nordic.

See my follow-up query in the chat, its first answer wasn't on point
Query:
"... my question was about countries that form an economic ecosystem largely unrelated to U.S. economic interests so e.g. would have most trade among themselves as opposed to trade with the U.S."

Part of Response:
"The Nordic countries (Denmark, Finland, Iceland, Norway, and Sweden) form an economic ecosystem that is relatively independent of U.S. economic interests, with significant intra-regional trade:"
And then it gives trade stats that seem to verify the claim.

Query:
"To the extent there is a Nordic economic ecosystem, is there an index or some other means that characterizes the share price history of participating companies?"

Part of Response:
"These indices provide different perspectives on the Nordic economic ecosystem:
The OMX Nordic 40 focuses on the largest and most liquid stocks across the region.
The STOXX Nordic TMI offers a broader view, covering a larger portion of the market.
The NASDAQ OMX Nordic 120 provides a middle ground, representing a significant portion of the most actively traded Nordic shares.
"

Query:
"Is there another example of such economic ecosystem or is Nordic the best?"

Part of response:
"The Nordic model remains the most well-documented and studied example in the given context."

Another response about Nordic countries:
"Exchange-Traded Funds (ETFs):
There are several ETFs that provide exposure to Nordic markets and are accessible to U.S. investors:
Xtrackers MSCI Nordic UCITS ETF: This new ETF, launched in February 2025, invests in large and medium-sized companies based in Denmark, Finland, Norway, and Sweden. It tracks the MSCI Nordic Countries Index.

According to ETF Database, there are 6 Nordic ETFs traded in the USA as of March 2025.
"
It also lists info on mutual funds, FNORX is the simplest to find info.

More digging is required.

Also of course, there are individual stocks to be investigated.