I get the point about "remembering it" if it's a one time thing. DS8 barely remembers our WDW trip when he was 5; DD11 barely remembers our
Disneyland trip when she was 6. Certain bits stuck with them, but definitely just bits.
Putting WDW off a few years means not dealing with strollers and/or necessary naps. That alone would be worth waiting for, to me! We are the weirdos who did not bother with strollers even for a (young) pre-schooler. We piggy-backed him now and then, but he mostly walked. That's a rarity, I will say. My niece was 4 for the trip when DS was 5, and she rode in her 1yo sister's stroller a couple of times, but walked 99% of the time and was fine. From the looks of it, a lot of people still use strollers for 4yo kiddos. It kind of depends on the individual 4yo's stamina, I think. She has it in buckets. Similarly, she was done with afternoon naps, as was DS (he was in pre-k, having just turned 5). They both slept longer in the morning than normal, but we never needed to get back to rooms to nap. If either child will need a nap - the cruise will be so much easier.
I think there is plenty of magic on a cruise!! Sure, there are lines to meet characters some times, but generally better than the lines at the parks; and you might also happen upon one who interacts with your child in a fun and unique way. Heck, even I had that happen with pirate Stitch as an adult! Plus the character breakfast is included, and you can get advance tix for princess meet n greets, and you can see a few character-based shows super easily (on a 7night). Plus you have the added "magic" of the ship itself. My kids think cruise ships are AMAZING.
If relaxation is what your husband wants, a cruise is surely a better bet than WDW. I love the parks, but there is (almost) nothing relaxing about the parks, and even less with a pre-schooler.