We have the UC Pro step thru in the queue, absolutely.
Doing fat tire + step through is definitely harder than it looks. From the Sondors picture, they are obviating the stress problem by having a single continuous chassis. Funnily enough that's actually how RM does the cargo bikes too...(very similar load and roll characteristics). Sondors appears to have put in the $$ and time to build something custom that works. That is awesome.
Question - FS versus Fat Step thru ? Tell me more about that.
It's all about comfort, and what is more important, I think, that with really fat tires like the Sondors 26" X 4.9" if you put in the very best seat suspension seat post like the Kinekt, and the best gel seat, you are getting really close, but a full suspension is still safer for keeping the wheel planted on the ground. So I guess it depends on what type of riding you are going to do.
However after watching this video, this is a hard tail fatty doing jumps normally only a FS MTB would do, it makes me wonder.
Big tires give you stability and safety and some extra built in suspension, with your new ultra, driving bigger tires is not an issue, you might lose some run time comparatively but with the newer 52V it shouldn't be a huge issue. I think 52V are perfect for fat tire bikes, they need it.