Return Flights from Dubrovnik to NYC or PHL

My goodness, you must be frustrated! If you are going to consider taking a separate local flight and then your return flight, why not consider a short flight to Rome or Milan and home from there? I am going on this tour in Sept (I can hardly wait!) and then following it with a back to back tour that starts in Rome. I found it pretty easy and not too expensive to fly from Dubrovnik to Rome.

I think you would find more flight options from either Rome or Milan that would increase competition and perhaps lower the cost.

Can you give yourself some wiggle room with an overnight in Zagreb? That would prevent any complications.
Have you checked Lufthansa?

We're flying Lufthansa from EWR to Ljubliana. They don't have anything reasonable for us on return from DBV ($1,600 EUR). We would really like to have the flight home scheduled on Saturday, so any day-of delays would ideally still get us home on Sunday. I really don't want to be flying home on Monday. We both work.

Would we have to go through customs or other delay when flying to another EU country like Italy? I've never been to Europe before.

Edit: I did look at flights from DBV to Rome and Milan but the times are all afternoon. There is an early morning flight from DBV to ZAG on our last tour day.

ZAG to DBV is just a short hop, so it would probably be fine. Probably being the operative word there.
1600 is not a bad fare. 1800 is getting up there, but for the convenience of a direct flight, I'd strongly consider it.
One other place to check is https://www.croatiatravel.com/. I used it once, but they did not really seem to have any secret flights you could not find elsewhere, nor significant savings.

The $1,600 (Lufthansa not direct) and $1,800 (United direct) are per person. My other flights were less than $800 each including seat selections and a checked bag, so in comparison they seemed exorbitant

We were going to take this tour in 2020. Figuring out how to get home from DBV was tricky. We ended up booking an Easy Jet flight to Amsterdam, with a one night hotel stay there and a flight home to Chicago the next day, non-stop on KLM. The 2020 tour of course was canceled. We re-booked it for June of this year and found the same issue. It's not easy to get home from Dubrovnik. So we again booked the Easy Jet flight to Amsterdam, this time planning to spend two nights in that city before flying home non-stop on KLM. The Easy Jet flight was super cheap. Even after I added a checked bag and upgraded the seat to allow a bag in the overhead bin, the fare was only a total of $95 per person. Of course, I was booking way in advance. We ended up having to cancel that tour because it conflicts with the anticipated due date of our newest grandchild, but some day when we re-schedule, I'll probably do it the same way. It's easy to get to central Amsterdam from the airport, and we like KLM as an airline.

I have always paid over 1000 per ticket for flights to Croatia, but I am picky about scheduling. This year things have been erratic in price and schedule. With ample time, you can save money, but it really just depends on your risk tolerance.

I would be willing to pay $1000 each way, per person. $1800 for a one-way flight per person just seems a bit pricey

You may find there is a better option if you are willing to stay an extra day or two. When I took this tour, I could only find flights with two connections on the day the tour ended. Two days later, there was a flight with one connection in Vienna on Austrian. If you are willing to use an extra vacation day, you may find better options.

I would be willing to pay $1000 each way, per person. $1800 for a one-way flight per person just seems a bit pricey

Why are you looking at one-way flights? Use the multi-city option when searching for flights. One-way tickets are nearly always significantly more on the major airlines. For example, search for Newark to Ljubljana for the first leg and Dubrovnik to Newark for the second leg. I just did this for random dates in May and found multiple flights for under $1000.

Because when I first booked this trip 5 months ago, it was significantly cheaper this way. My outbound flight to Ljubljana hasn't been impacted

I am taking the Adriatic tour in September. Our plan is to fly from DBV to Frankfurt on the day the tour ends, Lufthansa has a late afternoon flight. We will overnight at a Frankfurt airport hotel and then fly home (Orlando) direct the next day on Lufthansa. You could do the same on Lufthansa or codeshare on United.

Because when I first booked this trip 5 months ago, it was significantly cheaper this way. My outbound flight to Ljubljana hasn't been impacted

Ugh. How frustrating. I don’t suppose that your flight to Europe is refundable? If it is, you might be better off to get a refund and start over with a single ticket.

Unfortunately it's non-refundable. When I booked it said I could change, but I called and they will only allow me to change the date

I’m not certain where you’re at with the airlines at this point, but flights to Croatia out of Berlin are dirt cheap. I found I could get a ticket from California to Berlin for $750 Roundtrip and then catch a $50 flight to Dubrovnik, Whereas booking Roundtrip from Cali to Dubrovnik costs $1300. You do need to give yourself extra time to be sure you can make the connecting flight. You also should check Skyscanner.com for flights to Frankfurt, which is Lufthansa’s base and may offer more options for you.

And, you probably already know this, but if the airline tries to change the airport from what you originally booked, or your flight by more than 2 hours, you have the right to demand a full refund. This rule applies even to “non-refundable” tickets. The airlines will try to tell you it is not refundable but if they change your airport or go beyond a 2- hour change, it definitely is.
PS. I just now checked Skyscanner.com for a one-way flight from New York JFK to Dubrovnik on Sunday May 16 and found a fare for $360! Just an example, but maybe you can find something that will work for you and cancel your existing ticket for a refund.

I flew DBV to ZAG and then to PHL. they were booked separately.

most flights to US from EU leave early in the morning and that is the case from ZAG - so if you have to take a day flight DBV-ZAG, you may need an overnight stop .

(that said, ZAG old town/city center to airport is 20 min, Uber and taxi are super easy to find even super early in the morning and inexpensive. zagreb old town is nice.)