Not a crazy idea at all! Disney used to sell a land & sea package but stopped selling that way a few years ago. Most Dis-ers would vote for doing the cruise last but of course it's up to you.
I'm a
DCL junkie and have to say that the best $$ saving plan is to book the cruise uber far in advance. I usually try to book 2 yrs in advance or on day 1 of price release, which seems crazy to non-cruisers but I'll explain why.
When DCL releases their dates/prices newly available to book, that is the cheapest it will be. As the ship fills, the price goes up. (A few exceptions for certain sailings that are not selling as quickly as they'd like.) In general, any time schools are closed, the prices will be much higher.
As for the new ships, they are selling very well so I can't imagine any discounts coming.
For example, back in Jan I checked next school year's calendar and saw our kids have off a 4-day weekend for a Jewish holiday + teachers' inservice. I quickly booked a 3 nt cruise and friends of our (new to DCL) booked also. Within about 8 weeks, their fare (no kids) increased $700 and ours (2 kids) went up around $900. Crazy! And this is for a 3 -nt Sept cruise, which before the new ships would be one of the last to sell out. Very off-peak time.
I can't compare to NCL but we have sailed Carnival 3x and RC 2x, and DCL service just blows them away. It's not the characters or free soda but all the Disney touches, the shows, rotational dining, and lots of Disney magic that we miss on other lines. We walk into the dining room to smiling CMs who remember my name & know that dh & I like ice tea. They cut my son's meat and crack his lobster.
The focus is on having a great time with your family, not about drinking, gambling & smoking. kwim? Sure people drink but it's not like what we saw on other lines. DCL doesn't have the alcohol & casino revenue that other lines do; there's no casino and have a larger % of passengers that are children. So that's another reason you pay more for DCL.
Call me crazy but I don't even compare prices with other lines any more. Sure I can get a cheaper 3 nt cruise out of PC on RC but honestly, I'd rather go to work and save the vacation time for a cruise I
KNOW we will have an awesome time on. Just like I can visit Six Flags for a lot less than what it costs at MK for a day but if someone had never been to MK, they might think SF is a great theme park,

& not even know what they're missing.
You don't say when you're looking to cruise but I'd find a date/price that's doable and book as soon as you can. The price might just go up if you put it off.