Well your thinking in the right direction but in my opinion using the wrong tools as to making things work a lot easier.
To add context when our team switched to Unreal 4.0 we decided that we need a better solution as to our player model requirements as a replacement to what Epic was providing at the time. As a development tool we were using Daz Studio and Genesis 2 but with improvements to Genesis as a framework building resource we committed the app to our development pipeline as a preference. Over time the solution improved but more than whats needed info wise.
As a solution to your problems DS and G9 checks the boxes of at least building the resources and the required framework designed to fit you needs.
Maybe not? At the very least you will gain some insight as to how custom characters introduced by you clients could be developed using a framework that already serves the need of the end user of unique characters.
The trick that makes Genesis work is it’s the base for all characters made by Daz3d when applied to a working frame work and the linchpin falls into you need for a single skeleton solution via use of what they called Injectors, which is another name for full body morphs. As for the example.
In this example the character morph is targeted to the Genesis 3 and then scaled via bones to the correct scale. From there you have option as a resource you could use the bones as a layered additive for size and using the shape as an injector make any kind of character change you wish. The trick is to load the character morph first then layer in the additive second so it all behaves as in the demo video.
I would say first step is to first develop the asset pipeline so it will work with what ever framework you decide.