My opinions:
Deer Lodge County, Montana - Yes
This is a tough one, as just like rural Maine or rural Minnesota, it could be prone to a rural populist Republican swing in the near future, but I'm thinking it can hold out, as it just needs to reach one more election to achieve 100 years. It voted for Biden by 7.7%, so it isn't unreasonable to think this.
Costilla County, Colorado - Yes
While this is another county that is very prone to seeing a Republican trend in future years (even more so than Deer Lodge), it just was won by Biden by too high of a margin to think this is possible. It only needs to vote Democratic for one more election as well (2024) to reach 100 years, and it's tough to see it swinging Republican all the way from its 2020 result of D+27.2 by that time.
St. Louis County, Minnesota - Yes
This county is anchored by the city of Duluth and probably will stay Democratic for the two elections it needs to reach the 100-year mark. It also voted D+15.6 in 2020.
Carlton County, Minnesota - No
While this Northeastern Minnesota county also only needs to vote Democratic for two more elections (up to 2028), it's a very hard sell to see this happening. This is especially considering how close it already is, the lack of a large city, and how it has trended. This county went from Obama+26.1 in 2012 to Clinton+1.7 in 2016, and in 2020 it returned a result of Biden+1.5. This is difficult to see occurring.
Lake County, Minnesota - No
This county is similar to Carlton County except that it is slightly more Democratic (D+3.5 in 2020), but also has to stay Democratic for one more election (up to 2032) to reach its milestone. Don't see it happening.
Douglas County, Wisconsin is maybe. I voted no on the poll
Agree on a lot of this, and your analyses are quite good.
For Douglas, on which you were undecided, I'd go yes on Douglas County, since it voted for Biden by nearly double-digits and has just two races to go.
Generally I'm more bullish than you on rural areas, it seems. I don't think that rural areas will necessarily continue their rightward swings once Trump leaves office.
I nearly agree with Carlton that given that Carlton went blue by just 1.5 points and actually swung to the right in 2020. But I don't think Trump 2024 or a non-Trump Republican will keep up Trump 2020's performance in the region, which I believe was the GOP's peak in some places of the region for a while. Carlton, yes, just has a few more points to go, but also just a few more races. Barring a GOP landslide in '24 or '28, neither of which is likely, I think it'll cross the finish line, if just barely. Not saying it's definitely yes, but I will vote yes on the poll to be bold and back my beliefs on rural areas' post-Trump trends.
Lake County is admittedly unlikelier since it has one more race, which is one more chance of the perfect GOP candidate coming and winning over Lake, which I think outweighs it voting 2 points to the left of Carlton in 2020. Unlike Carlton, however, it swung leftward (though it trended rightward) in 2020. And as I said in the previous paragraph, I suspect a leftward swing in these counties now that the perfect candidate for them (Trump 2016 and Trump 2020) isn't on the ballot anymore.
Which results in my voting "Yes" for all 6 counties.