Seems as though you've made up your mind but I'll add to the discussion for whatever its worth to you.
I have one 10ft rod, which is a 10ft 7wt that is used for chasing Great Lakes Steelbows. It's darn near perfect for that application, but it really is a stick that has one niche and as Gator mentioned, is excellent with line management. I know the 10ft rods are all the rage right now, but based on your own description, it will get limited use.
I fish medium to large size streams with a 490 and throw everything from small to larger dries, single nymphs, double nymphs,weighted nymphs and streamers with it. I have no issue with line management or covering a lot of water with it. It's darn near the perfect stick. When I am fishing smaller water, I'll go down to a 486 or 489. The caveat to this is that my 490's are faster action, whereas the 486 and 489 are medium to medium fast and are probably better suited for dries and single nymphing.
If you are looking to add a stick to the quiver, maybe think about a 486 or one that you will get use out of if say you want to put somebody else on a rod, or one that you will find greater use from. A 10 foot rod, I'd have to imagine would not be ideal for dry flies for example, whereas a 486 or another 490 would be a better choice for a greater variety of bug types. Looking at your current lineup that's what's missing--though my disclaimer is that I don't fish in your area and am not familiar with your waters or the best suited sticks.
Don't want to shy you away from a 10 footer if you think it will add a dimension to your game, but I've read a lot of folks who have got into the 10ft game for trout who end up selling them because they are more one dimensional tools. Although they do seem to excel in their space.
I'm of the opinion that a good guide or guided trip can be worth its weight in gold if you are looking to learn new techniques or water.
Lastly, I too have been adding to my quiver over the past couple of years and am suffering from new gear syndrome. Have always been a gear junkie
One thing I've found is that while it gives me joy to fish a variety, I sometimes find myself having to concentrate when casting some of the rods as the action and performance varies. Versus when I had one rod that I got very comfortable and proficient with, I could just get out and fish with very little thought to what I was doing. Its a tradeoff and I have no regrets. Just adding additional perspective.