Thank you everyone for the replies.

JFK - FCO with a 1.5HR layover in LHR is one ticket with British Airlines. They had a RT fare sale for $260 otherwise I probably would have flown into a different airport.

I would be adding a separate ticket on Alitalia to Croatia. I guess we will do Rome first and then move on to Croatia.
I understand the ricks of flying on two tickets but for certain airports 90 minutes is more then enough.

Ive seen a lot of posts about how slow FCO can be and I don’t think risking it would be ideal since there is another option.

Doing it this way kills the final night as the flights to/from Croatia are only in the evening around 6 pm. So the evening of the 19th we fly back into Rome and then the morning of the 20th we have to fly back to JFK basically killing the final night as once we land and clear customs in FCO it will probably be 7.30-8 pm and our return flight home is at 8.30am. Will probably stay at the airport hotels that night.