Great photo. I experienced just what you experienced about exposure. Here is what I found works great.
I use the ZWO ASI 120mm mini with the 30m ZWO mini guide scope. However the key to this success was the ASIair Pro. It does it all. PA, guiding, camera control, etc. Yes its about $300, plus guide scope/camera and for most uses you need ZWO brand accessories. However, your Nikon D7500 is supported, as well as the iExos 100.
No polemaster, no polar align scope, NO computer, no USB hub. It uses an app running on your iPad or Android.
To begin with and save some $, I got the Pro, but no guide camera/scope. The Pro will use your D7500 and telescope to do a spot on PA. Takes maybe 5-8mins. No 2-3 star alignment. Then it will go to your target, dead center. With a much better PA, your exposure time should increase, however you are NOT guiding. If you later add guiding, I have gotten 3-5 minutes with my Nikon D5500.
One caveat, your iExos 100 must set up to use a serial connection and a serial cable to the Pro. You no longer use ExploreStars, (and it won’t work). You use the Pro app.
Just another idea. There are several Pro users on this forum. Harry
Vero Beach, FL
Mounts: ES iExos 100-PMC Eight
Scopes: ES ED80CF, ES AR102, Meade ETX 90 EC (Deforked)
Guide: ZWO 30mm Mini, ZWO ASI120MM-Mini
Software: ASIair Pro, iPad Pro, MacOS, Starry Sky Stacker, Affinity Photo, Lightroom, SkySafari Plus
Cameras: Nikon D600, D5500
Misc: ES USB Power Bank, Rigel QuikFinder, 0.8x Focal Reducer
Filters: Optolong L-Pro, L-eNhance