Have you considered going by train to Venice? If I were going from Civitavechia, this is what I would do. It is a lovely train ride in Italy. I would even consider a stop in Florence or Verona if time allows. Flying is not as smooth as it use to be and it is a short distance. Instead of being 3 -4 hours in advance at Fiumicino, you could already be confortable at your train.
Good information about trains in Europe you can find on internet under the posts of the guy of seat 61 (or 65 ?🧐 not sure).
Ivi