Are US ETFs worth the (estate tax) troubles?

Ok, I have sourced usefull information, I think.

tl;dr: You are not “US citizens, US permanent residents, and other clients who hold a 9 digit US Social Security Number, or US TaxID” then there is no reporting, and there is no freezing for the IRS in any case.

I asked IBKR:

And they answered:

The question is: Do my heirs still have to file if the IRS doesn’t know and it’s below the current exemption from the treaty?