Flying from US to Dubrovnik

Croatia Airlines is fine.
When I have flown to Croatia, I had limited time and wished to be on one ticket. I was able to get there four times with one painless layover, so I would try that first. If you have time for a stopover, search Skyscanner for low cost carriers that offer flights from whatever major city is your best stopover—where are you flying from?
There was once a seasonal flight from Philadelphia to Dubrovnik, not sure if it is still in effect, or if May would be too early.

United has a direct Newark to Dubrovnik summer route

We are starting from Columbus, Ohio so not looking to begin any further west than Chicago. We normally fly to Philadelphia to begin a trip to Europe but Charlotte and NY are fine. We would like to connect to Dubrovnik via one stop in Europe. Choices are London, Dublin, Paris, Rome, Munich, and Frankfort from what I can find. All the European flights are on Air Croatia or Vueling (Dublin is Aer Lingus). I need a minimum 2 hour layover to allow for a late arrival from the U.S. I am not looking for the cheapest way, just the most reliable, with only one stop in Europe. Just wondering if seasoned Croatia travelers prefer to go to Dubrovnik via a particular European city because they’ve had bad experiences elsewhere. Would love to take that seasonal Newark flight, but we’re 4 days too early and it’s really expensive. Thank you for your help!

If you can get to and from Philly, you can do this trip on Lufthansa through Frankfurt b both ways. It will be pricier than when we last did this.

I try to avoid flying through London, less stringent about avoiding Paris but it's not my favorite layover either. Amsterdam and German/Austrian layovers have always been fine.

We had a layover in Zurich from Boston to Dubrovnik in 2019. I forgot what airline we used, possibly Swissair. But I know from Zurich to Dubrovnik, we flew Croatia Airlines. No problems transiting through Zurich. Very easy connection.

I found flying to Berlin and buying a separate ticket to Dubrovnik was about $500 less than buying a ticket all the way to Dubrovnik.

In October 2019 we flew Miami to Barcelona via AA and continued onto Split via vueling's low budget partner ( all on same ticket so once we checked luggage in Miami next saw it in Split). We could have saved about $50 each with purchase of separate tickets via several carriers departing from Barcelona, but desired the seamless transition of the luggage.