Wow. A lot to unpack! Here’s a preliminary go on your inquiry on Flying Wings Silver Foxes (sometimes ‘SF’ here for short) and the Acrobatx.
I am familiar in varying degree with all of the kites you refer to, and here are my brief impressions:
The Flying Wings Silver Fox 2.3 and 2.5 are similar. As one might expect, the numbers in the names denote wingspan (roughly, in metres). They are both generally fairly largish in feel in flight, and require larg(er) hand/arm movements. Large(r) presence in sky. Very capable. Older design approaching 15 years or so.
The UL versions have pretty much the same sails as the standard versions, with a somewhat lighter frame. In my experience they have only slightly lower bottom ends, compared to, say, Benson or Jest of Eve models in comparing STD and UL versions.
The 2.3 Pro: a later refinement of the 2.3 in weight, construction, sail and bridle. Still available from a few retailers. Have, and like, the STD; lusting after the UL
The 2.2 is a variant (approximately 2018) incorporating Lam Hoac’s further tweaks incl layout, shape, bridle etc, and features such as ‘claws’ on the LE for YoYo manoeuvres. I find the 2.2 UL a more sleek machine flight-wise in comparison to its predecessor. Unfortunately, I am not familiar with the 2.2 STD but I would expect similar handling but in higher wind. As in other kites, some tricks may be easier (or at least require slightly different inputs) in one version over another due to weight distribution, etc. Google the review.
Acrobatx: also same era as the early SFs but a little smaller, at closer to 7 feet. Heavier build, using more economical materials, with pultruded carbon frame and a nylon sail. No leech line so can be noisy (unlike the SFs, which can be adjusted for this). Flight-wise, quite manoeuvrable. Trick capable but a bit faster than all the SFs generally, in my experience. Not as refined. Smaller hand movements required. Comparatively less pull unless windy and depending on bridle settings. Good bang for buck, especially at sale prices I have seen. Good upgrade from Quantum trick-wise imo. See the review in KiteLife in approx 2006. The UL version has a much lighter frame and sail, and more of a finesse feel, but lighter handling as many ULs are. It is more $$ due to materials etc. Carbon-wrapped tapered spars in the frame.
Looks and appearance in all these including colours: being subjective, I can’t comment.
Lines: I have several sets but generally use a set of 50# x 75 or 85’ on the ULs, or sometimes 90# in stronger end of the wind range. For STD kites I usually use 90#, sometimes 120# or 150# in more wind.
In summary, all of these are fine kites in their own way. TBH, to varying degrees, they might all be considered an upgrade over the Quantum, which is a fine but basic kite in my opinion. (I can’t help but think that more time with your E3 is in your future, as it, too, is quite a capable kite.) While I see and appreciate the place the Quantum fills, comparatively it feels rather … clumsy to me. If that’s not too strong a word. (I still like mine though!)
I hope the above is of some assistance. Any Qs, ask away. I am confident that there will be other views on this but this is my perspective. (Just a part-time recreational flyer with too many kites, if that is even possible.)