Italy/Spain trip

Not sure about the flights to San Sebastián airport, since they only have flights to Madrid and Barcelona (Iberia and Vueling respectively), plus a couple of seasonal to London and Maiorca. You will probably have to fly to nearby Bilbao (BIO) which has flights to Italy. That would spare you an extra layover and I'm sure flights to BIO are cheaper, since it's a busy airport.

As a general rule I would recommend that you reverse the itinerary and start from Venice, then work your way south to Rome.

Vueling, a Spanish airline part of the IAG group (British Airways, Iberia, Vueling) flies from Rome-Fiumicino (FCO) to BIO.

If reversing is not possible, starting this year a Spanish low cost airline called Volotea will start a seasonal flight from Venice Marco Polo (VCE) to BIO, not sure if daily however.

Check dates and prices in the websites for both Vueling and Volotea.

Venice first was what we originally thought but assumed there would be no flights from Miami/Tampa Florida to Venice. should have added that, sorry

we want to visit Bilbao too so thanks for that and other input!

Not sure if there are too many flights to FCO either from Florida. Maybe Alitalia flies to MIA or MCO. Check Alitalia website.

VCE has flights to nearly all European hubs ((various European airlines) and in summer, to JFK, ATL (Delta), EWR (United), PHL (American).