The one thing I would add is an eyepiece which provides the widest possible field for your telescope.
With 1.25" eyepieces this would be an eyepiece with a focal length from 20mm to 26mm and an apparent field of view between 62° and 70°, such as:
- Explore Scientific 26mm 62°
- APM UFF 24mm 65°
- Tele Vue Panoptic 24mm 68°
- Pentax XW 20mm 70°
If you want a wider field of view then you have choose a lower focal length - 17.5mm and 76° with a Baader Morpheus, for example, and 16mm and 85° with a Masuyama. These two will give a slightly smaller true field of view.
Your zoom + 2x Barlow should cover all other situations, so I wouldn't go and get any more eyepieces right away.
Later on you might want to buy a fixed focal length eyepiece: maybe you find yourself often using the zoom at 9mm with the 2x Barlow, so you might get a dedicated 4.5mm eyepiece to use instead of the zoom for those situations.
Use the handy FoV calculator at astronomy.tools to compare eyepieces and see the visible area given by each one.
You'll see that all of the eyepieces show roughly the same real area of sky (the 24mm/68° shows the most, and a 40mm Plössl with 43° would show more still).
Where they differ is in the apparent field of view and the size of the image when you look through the eyepiece.
Your Orion zoom has a FoV of about 40° at its 21mm setting. A 24mm eyepiece with a FoV of 68° will give you a low power (about 20x magnification) view which is twice as wide as that given by your zoom.
You mention viewing under light polluted skies as well as darker skies, and the wish to see nebulae and galaxies.
Light pollution and galaxies are incompatible, unless you use image enhancement technology of some kind (night vision, live stacking, or full-on astrophotography).
Except for the biggest and brightest galaxy - Andromeda, M31 (the core of which should still be visible beneath all but the worst inner-city skies) - your search for galaxies will be a frustrating one.
Even under dark skies, seeing galaxies with the naked eye is not easy, requiring the use of averted vision and memory in order to see any detail.
Some nebulae are visible from the city. The Orion nebula, M42, is very bright and always worth a look, no matter where you are or how many times you've seen it before. In the summer the Triffid nebula can be seen in the southern skies, and the Swan (aka Omega) nebula comes out with the help of a filter.
There are also some bright planetary nebulae which you can see from the city. The Saturn nebula comes to mind as one I've seen under light pollution.
Generally, though, the best sights for the city astronomer are:
- The inner planets and Jupiter and Saturn
- The moon
- Bright star clusters
- Bright double stars and carbon stars
(Disclaimer: my urban skies could be a lot worse, and my telescope could be a lot smaller, so what I can see won't necessarily be the same as what you can see)
Also, you could get yourself a couple of books. A paper star atlas is a nice supplement to an electronic one, especially for those trips to darker skies.
Turn Left At Orion is a classic book which is frequently recommended. Nightwatch is another.
Get a red light as well, to preserve your night vision in the dark.
