Some more info for your consideration. Happy to elaborate on any of my points, if helpful.
Paris and London: My husband and I did a Paris (5 days) and London (4 days) trip independently in 2018. (We included Ireland for 10 days, too.) Easy direct flights from the east coast. An easy, two hour Eurostar direct train from Paris to London. We worked with our TA on amazing hotels in Paris and London, some private tours (Louvre, Windsor Castle), some small group tours, and a lot of planned independent touring. We successfully enjoyed a mix of touring, relaxation, wonderful food with the help of our hotel Concierges, and very easy travel. (Note: It was our first trip to Europe.)
Actual cruising: Two Celebrity cruises. (Caribbean and Alaska) A partial Viking Trans-Atlantic coupled with a cancelled Mediterrean Odyssey (MO) as the COVID shutdown occurred in March of 2020. (Miami to St. Martin to mid-Atlantic until a turnaround and return to the states without ever reaching Europe.) An actual MO cruise in March/April of 2022 with preceding independent travel in Rome, preceding Viking extension in Venice, and post-cruise Viking extension in Barcelona. So, I'll focus on summarizing the 2022 trip.
Even with all of the masking and vaccine card reviews, our independent travel in Rome was simply amazing. We worked with our TA to find a wonderful hotel, privately guided tour of the Vatican, small group tour of Ancient Rome, and independent touring of the other famous sites and The Borghese Museum. An easy train ride to Venice to join the Viking pre-extension which was a disappointment. (The disappointment surrounded the unexpected change of Viking hotels, the actual Viking hotel, and the on-site Viking hotel representative.)
I can't say enough good things about our wonderful Viking cruise and the great ports that we visited. We filled our port days touring with a mix of included and optional excursions. A bit of independent exploring, too. (Touring bubbles were in the process of being lifted on our cruise.)
A disappointing post-extension in Barcelona. (The disappointment was the Viking hotel location.) But, a wonderful privately guided tour of Sagrada Familia and Parc Guell arranged through our TA.
Note: Because of the state of flux with country regulations, testing requirements, and in-flux transportation challenges, we were aware that we might be disappointed in the Viking extensions/hotels, but, made the decision to proceed.
While I can't reference the port in Rome, I can comfortably say that the port in Barcelona would be really, really easy to handle independently. The infrastructure, the signage, and the plentiful taxis at the entrance/exit location justify my statement.
Compared to our Paris/London trip, our MO cruise felt like we were constantly on the go without the ability to audible from touring very often. So, from my perspective, a potential difference in the type of trip that I would pursue for a honeymoon. Plus, the bonus of the large cities that you are considering are really easy for independent travel.
As for passenger demographics, I'm a very active and fortunately retired 60 year old. On my two Viking cruises, I felt very, very young. (On many excursions I had to make a concentrated effort to slow down and not make other attendees with walking challenges uncomfortable even though I wanted to speed things up.) Would I have felt out of place on a Viking ship 34 years ago for my personal honeymoon, absolutely! If I wanted to take a Med cruise for my honeymoon, however, the age factor would not stop me from considering Viking! Nor should it stop you. (Nice food and a fabulous crew are great for any age.)
My advice: Prioritize the places that you'd like to have as the basis for your honeymoon memories. Identify the "things" you want to see at those places/cities to help make the determination of the appropriate length of stay needed. Use those pieces of data to help make the decision to cruise or to independently take a land tour. (Or, as some others have posted, a group guided land tour.)
It sounds like you'll have a wonderful honeymoon with whichever direction you take.