True, there is not much difference in the field of view between the Pan 35 & Nagler 31, but I found that the view in the 31 was more pleasing with the stars as pinpoints further towards the edge, and easier to look through. It also complements the 17E very nicely. I can scan the sky with the 31, then zoom in a bit with the 17. As for the 13E, it's not much of an increase in magnification from the 17, so I don't use it much.
Bought the Delos 8 because I had heard so much good about its sharpness, and it is very sharp, but having a narrower fov than an Ethos, it doesn't really fill the gap between the 13E & 4.7E for me. It is a bit low power for globulars, and more so for planets. I think that I might use either an 8E more, or if I follow my previous post, replace both the 8D & 13E with a 10E. Ideally I would purchase both 10E & 8E to try, but they are quite expensive, unless found used.
I use the 6-3 zoom for double stars & planets on a driven mount, and the 4.7E for planets and globulars on a push-mount. The 4.7 is actually a bit brighter than the 6-3 zoom at similar focal length. Remember, everyone's tastes differ & what works for me might not be ideal for others.