I'd suggest you also spend time watching youtube videos/reviews of the ships you're considering. A ship can be beautiful and stop and the perfect ports for you, but still be disappointing if you don't enjoy the culture of the ship. Videos made by people who seems to enjoy the same sorts of things you do can help narrow things down.
Don't bother with a short cruise if you want a romantic experience. Shorter cruises on non-Disney ships has a reputation for being focused on partying.
I see a lot of recommendations for HAL, but I wouldn't choose that line and I'm in my mid-40s, because it has a reputation of being for an older demographic. I went on a Celebrity cruise a few years back and there was virtually no nightlife. Thankfully, we were with a group of friends and had a good time. I would choose NCL/
RCCL or Princess if extended family was going. I'm currently booked on MSC in their Yacht Club (a ship within a ship luxury experience) to see what that's like.
A few recommendations:
Book a pass to the thermal spa on your ship, if that's your thing. You get extra relaxation options in an adults only area that's also much less chaotic than the pool deck.
If you want an adults only pool area, not all ships have them. Make sure the one you choose does, if that's your thing.
If you're planning on a drink package and a balcony, know that room service drinks are not covered by the packages. If you choose a cabin near the stairs/elevator, it will be easy enough to pop out to grab another round. You can see on the deck plans what is near your cabin.