Suggestions for a "layover town" on the way to Athens

Assuming you are flying out of Erie, cheapest tickets are on UA/SAS. So that indicates Copenhagen as a good place to stop over for a few days.

Have you thought about taking a day flight from the East Coast? This totally eliminates the fatigue from not sleeping all night on a flight. Check out the major airports between NY and DC to see what's available. There are not a lot of them, but they do exist. Try AA or BA to start.

If you are considering a stop-over on the way to Athens you need to pick destinations that are non-stop from your gateway city so that you minimize your travel time. And then the same for onwards to Athens - pick a city that has non-stop flights from where you stopped off. I can commiserate with the sleep issue so I try to minimize connections which just adds to the "misery" of lack of sleep.

I'm not flogging a particular site, but it's easy to get a list of Arrivals at Athens airports. Here's an example: https://www.airport-athens.com/aia-arrivals I'd start there, and then think about frequency (for example, if your booked flight is cancelled), airline reputation, and price, plus baggage charges on bargain airlines. I don't know what time the tour starts, but we try to be in cruise and tour cities the night before our departure, just in case.

First, a great tip I learned on this forum: to find who flies where from a particular airport, look at that airport's Wikipedia.

Are you flying out of Erie to start the trip, or can you get to another airport easily? Looking at the Erie airport listing on Wikipedia, I see there are feeder flights only to Philadelphia, Detroit, or Chicago.

Day flights to London leave from Boston, Newark, JFK, Dulles, and Chicago. You land in London in the evening, so you get to your hotel and can go right to sleep. I haven't taken one, but many who have difficulty with transatlantic flights swear by them. However, for you this probably means a night at an airport hotel in your US departure city (these flights leave the US at about 8 AM).

If you are starting in Erie, you may want to fly through Philadelphia right to Athens. If you just do this 2-3 days before your tour starts, you'll be over the jet lag in time. However, the nonstop flight from Philadelphia to Athens is "seasonal" and I don't know if it's still going for your October travel.

Berlin is a fabulous place, but only a few nonstops go there from the US (now that Air Berlin is defunct), so that adds complexity. Amsterdam has year-round nonstops from various cities, including all three of the cities with flights from Erie. If you don't want to start in Athens, that's definitely an easy option.

Budapest to Athens, $35 to $100 one way on Wizzair or Ryan, non-stop in 2 hours....

I've seen some good prices on flights with layovers in Istanbul. It might be a good fit for going to Athens?

Of places you haven't been Amsterdam is a common hub. Frankfurt is another.

Barcelona has non stop flights to Athens

If you don’t want to add more time to Athens, then I would choose Amsterdam. The art museums are excellent and you could also visit the Anne Frank Haus. Charming, old world feel around the canals.
I like Berlin too but the airport in Amsterdam is so easy to navigate. Berlin’s is old and more difficult to deal with.

We have used Amsterdam as our arrival airport several times. There are hotels within easy walking distance of the airport. The railway station is right there also, so it easy to get to downtown Amsterdam in a half hour or so. The railway station is right downtown. Come out of the station and hop on a canal boat for a tour.

I'm a senior and I find the hassles of airports, transportation to and from, security checks,keeping track of luggage, and all of the rest of the essential stress, to far outweigh the alternative of sitting on one aircraft for two or three more hours, however sleepless.
On the other hand, if you want to see any of these stop-over cities for their own sake, then that's a much better reason to break up the itinerary.
To find short-haul budget flights in Europe, www.skyscanner.com

Amsterdam is great but pricey. I have just reserved lodging for Sept.