If you are considering a zoom eyepiece, the SvBONY 3-8 mm might be a particularly good choice.
My reasoning is that you probably have more need of a higher magnification than your 10 mm will provide, and not so much for magnifications in between what the 10 mm and the 25 mm will do. The 3-8 mm zoom spans the range from a bit more magnification than your 10 mm -- 150x -- to just about as much magnification as you would ever want to use on an 8-inch telescope -- 400x. *IF* the zoom eyepiece works for you -- and at f/6, some zoom eyepieces may have problems -- then it will provide plenty of choices for medium to high magnification work, which you will need, depending on seeing an on what you want to look at.
A 7-21 or 8-24 mm zoom does not give you the option of using powers that would be considered "high" for an 8 inch, on those occasions when you want them.
My comment about probably not needing magnifications between the 25 mm and the 10 mm is based on my own experience with deep-sky observing; 25 mm produces 48x and probably at least a one-degree field, which is useful for finding things, for looking at wide objects, and for looking at low surface-brightness objects. 10 mm would give you a magnification of 120, which is pretty much what I would choose for observing galaxies and many clusters. Your mileage may vary.
I am not saying that an eyepiece of 15 to 18 mm focal length would not be useful, just that it would be less useful than higher magnifications.
Again I stress, not all zoom eyepieces will work at f/6. Try before you buy if at all possible.
Clear sky ...