DazV5 is a technical standard. It refers to the technology behind the models - rigging, topology, materials, and baseline Bodymorph flexes that are standard among all my newest models.
DOAFantasy is a specific model type. It is simply one of two body-types, alongside the Curvy type, that is shared between multiple models.
DOAFantasy (DazV5 Prototype 3) is, as the full name suggests, built on the DazV5 standard. Same as the Curvy DazV5 body.
As to your point regarding morphs, it's a nice idea in theory, but in practice it isn't feasible for two main reasons:
Reason 1 is retroupgrading. Let's say I build a model for a very slim character, like Juliet Starling from Lollipop Chainsaw. She has morphs to take on the shape of all other DazV5 characters made to that point. Great. But now, a month later, I make a model for a bulky character, say Zarya from Overwatch. She has morphs to take on the shape of all other DazV5 characters made to that point, too, including Juliet. Great.
But what about Juliet? If she is going to have morphs for all DazV5 characters, then that means I have to go back and retroactively upgrading (EG "retroupgrade") Juliet to add the Zarya morph. Which then means I have to reupload the newest version, release it, and notify everyone that an update is available. And that's not to say that adding new morphs to models will break any project files they're currently being used in. And then how about all of the models made before Juliet? I have to update all of them too. And then I do this for every new model I release? I have over 11 pages worth of model releases on SFMLab, and that's with the last dozen or so models not even released onto SFMLab because I can't be bothered writing up proper release posts for them. I'd quickly reach a point where I spend longer upgrading all of the previous models every time a new model is released, than I spend actually making the new models.
Reason 2 is paltry in comparison, but that is the fact that Source only supports 128 total morphs. On the standard DazV5 set, I have only about 20 free morphs available. Considering that each character has 2 shapes - cleavage-breasts and bare-breasts - that means only 10 character shapes can be added retroactively. So even if I was willing to go through the hell I described above, it'd be for naught - only the first 10 models could be retroactively supported anyways.
It's a good idea in theory, and is actually one I am incorporating into DOAFantasy DazV5 Prototype 3, where all of the canon Dead or Alive body-types are built into the DOAFantasy models as shapes. But the reality is that it is a proper nightmare to maintain, and I am already running into severe problems with the outfit end of things for DOAFantasy, due to just how disparate the bodies are. Getting an outfit to maintain proper shape and rigging for both DOAFantasy's giant tits, and Marie's tiny tits, is not a trivial task.