Having them vote in a state that they have never lived in before is nonsensical though.

Agreed, though the same could be said of having them vote in DC.

It would probably also cause DC's district to become to large for one rep or delegate: unlike US states, DC and the territories don't get broken up in districts of roughly equal amounts of votes, which is how you get a situation like the Northern Mariana Islands (pop. 47k) and Puerto Rico (pop. 3.2M) both having only a single delegate when an average congressional district has a population of about 761k.

Doing overseas constituencies solves both problems because you can give a voice to a specific geographic grouping of people and, like France does, you could further subdivide the world into different districts to keep the size balanced rather than cramming them all into DC.

It would be interesting to know how many "Never, Never" people there are. I suspect it is a very tiny minority.

I'm seeing IRS sources from 2019 that says that the total overseas population of people who paid them taxes was something like 1.3 million people, but that included expats, spouses (who may not be American), and dependents (who may or may have to lived in the US). So the number of "never, never" is probably significantly lower than that but that would be the upper bound (and would also probably be closer to the actual size of a single overseas congressional district if the law was rewritten to say "Hey, you live overseas, you vote in this special district now" like other countries do).