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Use whatever process you feel makes sense and lets you repeat testing.
I don't know how you do your processing or how you want to work, there are an infinite variety of things you can do and "cheats" to get around some problems. For myself I have a recent physical disability that restricts what I can do and what of my equipment I can use, so I do things differently now to what I used to. 99% of my shooting is DSLR on tripod, 1% compact on polarie mount. So I guess the limitations I have there are things I overcome with processing. Eg I cant use my good tracking mount anymore to get long exposures so I instead take lots of subs to bring out the signal. I also have to use faster exposures to keep round(ish) stars.
All this is to say what I might suggest isn't going to be the only way to proceed and you can always try alternatives for yourself to achieve the same result.
First up zoom lenses are not as sharp as prime lenses and neither are at their best sharpness when wide open at fastest aperture. The optical elements get tested at the factory and need to be within a range of values. Parts with the best optical accuracy tend to end up in the more expensive faster lenses while those that aren't so good end up in the cheaper slower and kit lenses. A single lens will have around a dozen optical elements all with their own slight variations too. So each lens made gets tested too and must perform within a range of values. So no two lenses give the same sharpness and chromatic aberation figures. Even two of the "same" lens.
Because zoom lenses need to perform "acceptably" at all focal lengths the testing tolerances are much wider than for primes hence the generalisation that a prime lens is always better than a zoom but due to overlaps its possible for a particular zoom to outperform comparable primes.
Also apertures can be changed and likewise each lens needs to perform within a tolerance across a range of aperture settings. So a common photographic misconception is a fast prime is pin sharp at its fastest aperture when it won't be. You tend to need to stop down a little for the sweet spot. Then people complain because they bought an expensive lens expecting it would make them an instant famous photographer.
So when testing your lenses expect disappointments and surprises. During the day take shots on tripod at the sky through a neighbourhood leafless tree, fill the frame. This will give you a high contrast test across the whole frame of chromatic aberation performance. Fine dark random branches/twigs against a bright sky shows the colour fringing easily for all cameras/lenses. For astrophotgraphy this CA translates into an ugly colour edge around stars and you can see purplish looking stars in some people photos where CA was the problem. If you're using JPGs it tends to be worse but with raw you might be able to run through a raw converter and process most of it out .
Anyway, with zooms I stick to the extremes of its range, you always have to crop anyway when stacking subs. My main workhorse is a 70-200 f2.8 and its permanently set up at 200mm, fully focused and ready to take outside any time. So I suggest your 70-300 just keep it at 300 and test there. Being your longest lense you'll need to know how to use it for moon and planets , I guess your 17-85 @ 85 will be your most practical workhorse. When I started astrophotography I quickly found there is so much up there our eyes cant see but our cameras can quite easily so you can never have a too long focal length but remember you're trading off exposure time. Every photo I took I could see more interesting things like clusters and nebulosity so I started plate solving a good single and making an annotated reference of what else is in the shot I might like to go after the next night. Getting familiar with the constellations help your get framing good the next night.
During the day do a bunch of f-stop tests photographing street signs or shopfronts or print yourself a sharpness target so you can try to find your optimal aperture for sharpness on each lens at the preferred focal length.
For myself, my DSLR only ever gets used for astrophotography, I have other cameras for regular photography so I keep it set up on tripod with the 70-200 attached, it stays at 200mm and I have it prefocused since I never need to change focus and I'm careful to never touch or bump the lens as I spent several nights just taking test shots for focus and checking frames up close on computer until I was happy. You might be able to do something similar if you have lenses you dont use for regular photography but you find are good for your astrophotography. This also means I have my camera manually set ready to shoot. The goal was to not have to think about anything except dragging my stuff outside. But you may not be able to dedicate gear solely to astrophotography to have permanent settings. So I'd just a notebook you can keep your maximum and good settings in. By maximum and good , I've arrived at two settings for my targets I go after, "good" is where I want nice round stars so detail is sharpest, "maximum" is a slightly longer exposure time where I want to grab as much signal as possible but keep stars as close to round without obvious streaking. I keep ISO at one setting I am happy working with.
As I said at the start I work 99% dslr/tripod but all my above process works great for my compact polarie setup. When you push the weight and focal length recommendations of the polarie you start to get elongated stars. Its time consuming and boring to do lots of testing but it pays off. Then you can look at your processing, for myself I almost never look to get an image in camera, even solar and moon shots I often am looking to use several shots to stack. But its all up to you how and what you want to shoot and make sure you keep all your source files, in a year you will know so much more than now and will be able to make better use of the same shots. I'm in the process or reworking my workflow and looking forward to reprocessing some of my old photo sets.
Above all just enjoy it all and learn from, there are no set rules for anything, you can try different approaches and see how it works for you.
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