I think you are halfway to victory already. You know it will be different and you aren't expecting the moon. But I do think you might be pleasantly surprised. I thought Carnival's food was on the same tier as
DCL, and the staff was helpful and friendly (if a bit more stretched).
I think it mostly comes down to what is your favorite parts about cruising. If you tell me your favorite parts are Disney Characters, Disney shows and on-board entertainment and such, then you might be disappointed on Carnival. But if you tell me that your favorite parts of a cruise are relaxing in the sun, seeing different ports of call, enjoying the food choices and having a relaxing time with your family, then I think you will be quite pleased.
As for the older/smaller ships versus the larger newer ships, you probably know what your family prefers. Did you have a strong preference between the Dream/Fantasy and the Wonder? That probably tells you if you prefer bigger or smaller ships.
Here's another thing. I know you are considering Carnival to save money, and that is great, but if you and afford to take 20-50% of the savings of Carnival over DCL and put it back into activities (Spa, excursions, up-charge restaurants, etc.) you may find that you have just as much fun because you aren't tapped out by the cruise fare and can afford to splurge on some of the extras that maybe you had to skip on DCL. Just factor it in to the cost ahead of time so you don't feel guilty about spending the money. For example, (made up numbers) if a DCL trip cruise fare was 10K and the Carnival one was 5K, tell yourself the Carnival one costs 7K and use the extra 2K on activities that you could not have done because of DCL's upfront costs. You still save a lot of money, but you feel better about treating yourself to things as well.