Are you sure your idea of fairness is right?

Consider Mongolia, perhaps they want to match EU employment standards but can't afford do. What are they suppose to do?

Suppose a democracy decided on different employment standards, should EU ban imports from US due to EU having higher standards of employee and environmental protection? How big does the difference have to be?

Does your free trade "fairness" account for the advantages US gets from being a global center of finance?

For disadvantages Vietnam has from being a recent colony and having a devastating war?

Does a country with loads of natural resources, or elderly population, or struck by natural disaster change the calculus?

Is this a robust rule you would like to see implemented, or is it just justifying current politics?

It’s a robust rule I’d like to see implemented. Obviously there’s a lot of nuance and important details, some if which you highlighted, but the general idea is, that the goal should be to make a policy that’s “fair” in some way or another, and not just go “hur dur muh free trade” free-for-all. For example, I’d probably give Mongolia a bit of leeway as it’s a poor, developing nation to help them catch up, whereas I think US should be punished mot for the reasons you highlight, but for being a tax haven and maybe even for pollution.

But in general, my argument isn’t about some specific countries, but instead about specific products. A company in Mongolia selling to EU could either not give their employees medical insurance and sell widgets to EU for €50 + 100% tariffs, or treat their employees by EU standards and sell widgets for €100 and no tariffs. The ideal tariffs would exactly balance that (while still allowing Mongolian widgets to be cheaper because of lower wages because of lower cost of living.)

to go full conspiracy theory, standards of fair trade, e.g. labour standards, worker rights, IP etc, exist to cripple emerging economies, to contain their growth and limit their ability to compete. political spin doctors have wrapped this up in the veneer of fair trade, equal standards, to get ordinary people to take the bait.

there are ways to mitigate the growing pains of economic development but to prevent it completely is not a solution. nearly all industrial nations have, at some point in their history, gone through the whole process - discovering necessary safety standards, methods to encourage cooperation and where it fails, enforcement and legal repercussions remedies and punishment, etc... that really ought to be hyper-local and community-specific in their implementation. first world governments are fully complicit in preventing emerging nations from developing this themselves.

Factor those externalties - internalized for some manufacturers by regulations - that allow them to manufacture more cheaply into the import tariffs. Trade is not the same as a tariff-free common market.

This is exactly my position as well. Every country in the world needs to be held to certain minimum standards.

If a country pays its workers pennies and craps all over the environment, it sucks for the people that live there, but really it's fine, they're a sovereign nation, nobody's gonna send in the troops over it.

Let's simply charge them enough to take all the money they save / make from these practices out of their pockets. If they don't pay up, the goods can't cross the border.