Wandering the streets may be hard in Venice in December due to

https://en.wikipedia.org/wiki/Acqua_alta

Just chimming in about weather in December. I'm from New Orleans and lived many years in Houston. We've travelled to Germany and Austria three different times in December and it was so cold (but beautiful) that I was miserable the entire time. Be prepared with lots of layers and be ready to retreat inside.

We have been to all the cities you mention except for Barcelona.

If it was my itinerary, I'd plan the following:

Fly into Venice, 4 nights
Florence 2-3 nights
Rome 4 nights
Paris 5 nights
London the rest of the time and fly home direct from there.

It is kind of a pain getting home from Venice but there are direct flights to IAH from London and Paris. I like Venice to start your trip because it's small, beautiful, romantic, has no cars or scooters, with amazingly beautiful sights. I think it would be a wonderful beginning of your trip.

Good luck with your planning and congrats on your upcoming nuptials. Cheers.