Feed conversion in a commercial setting verses a home setting is comparing apples to oranges, in my opinion. I am not sure what your long term goals are, but even one pair of ducks can supply a heck of a lot of meat/eggs. There is too much variability in production between birds and too many other various factors to really be able to say what is right for you. You are not a commercial farm where things like feed conversion can be averaged over several thousand birds. For example, a flock average for a hybrid layer duck may be 225 eggs/hen and yet you may end up with a bird that lays 50 or 300. Does that make sense? Also, a commercial farm is going to be processing birds that are several WEEKS old, not YEARS.
What are your ultimate goals, meat or eggs? That will make a big difference right there as some of the lighter breeds are awesome layers, but not great meat ducks. If eggs are top consideration maybe a hybrid layer would be best. Do you want to sell a few offspring/hatching eggs? Then maybe something like Welsh Harlequins would be better. Do you like the taste of traditional duck or prefer something leaner? If so, maybe Muscovies or Scovie-hybrids would be good. There are just too many variables for us to tell you what is best for you if this is to be an ongoing project.
I will say this though, generally meat ducks are processed when they are only several weeks old, not years. I have never liked the taste of duck, so I can't say what is best or how much difference it makes in meat quality (in chickens though, IMO, age makes a *huge* difference). My brother though is a large commercial meat wholesaler and their ducklings are processed at like 7 weeks old. Again, I can't say how edible ducks that age will be because I don't eat them, but you might ask others. If anything, you might use them as breeders or to produce future breeders. Good luck!