Southeast Massachusetts to Nashville TN

When I lived in Belchertown MA we went to Nashville a number (30+) of times. There are 3 acceptable routes. (1) I81 as you suggested or
(2) through PA and Ohio or (3) through upstate NY and Ohio. All are about the same distance and depending on the time of day take about the same amount of time. It depends on what you want to see on the way.

Any recommendations for a 1/2 to 3/4 way stopping point for the night for both the way down and the way back?

Your route is reasonable. jplante4's route is also fine. If memory serves, the bit of I-84 through New York state was rather rough the last time I was on it (a year or so ago).

I-81 is in my opinion a much nicer route than I-95 for getting to Tennessee. I-95 through NYC, Baltimore, and Washington, DC is not exactly fun driving. You'd have to go over the Great Smokey Mountains to get from I-95 to Knoxville, presumably on I-40, and that's at least as bad as anything you'd encounter on I-81. There are hills along I-81, to be sure, but they're generally more of the rolling variety than anything. (No interstate is going to have worse than a 6-7% grade in any case.)

Having been hung up in NYC traffic on some very bad local advice, I might find a way west(I-84 maybe??) above the City and then head south on I-81.

We have traffic in Atlanta, but NYC has TRAFFIC!

Our route through there is I-195 to Providence, then I-95 to Old Lyme CT, CT 9 to Middletown then 66/I-691 to I-84. They just finished rebuilding I-84 through Waterbury and Danbury, so if you've been avoiding it, it's better now.

From there you have a couple of choices. You can stay on I-84 all the way to Scranton to pick up I-81 or take US209 down the Delaware Water Gap and pick up I-80 to I-81.

The roads around the NYC metro area are pretty dismal. The new Tappan Zee bridge is great, but the roads to it are still 3 lane and very crowded.

Done the north south trip 3 times now. Went 95 (over the George Washington) whole way once, NEVER DO THAT AGAI!. Went over the Tappen Zee the second time. Went all the way out to 81 and skipped the Tappen Zee the third. Skipping the Tappen Zee with the 5th wheel was WAY better!

I think your route is the way to go. 81 at least from Roanoke into K'ville is good with no bad hills. The only grade from K'ville to Nashville is just west of K'ville giong up onto the Cumberland Plateau which is not bad.

I came down 95 a couple weeks ago for the first time in a few years. We usually go 81. I used to come 95 just for something different but I my not do it again, it's not fun any more. 81 is way better