Here is a link to a wide-angle photo taken with the Nikon D850 45MP full frame DSLR & Nikon 18mm f/2.8 AF-D, f/2.8 for 30 seconds at ISO 6,400. The D850 was released in early 2017 and then Nikon released the 24.5MP D780 in early 2020.
You can see the D850's circa 2017 sensor captures the Double Cluster in Perseus, as well as the M31 Galaxy. Would the 3+ year newer Nikon sensor do any better with higher ISO performance? I'd be using a tracker for 5 - 10 minute exposures with a variety of f1.8 - f4 Nikon prime lenses wide open at ISO equivalent of @12,500.
Wow, that's a very blue image.
If you take a 10 minute exposure with an F1.8 lens at ISO 12500, your image is most likely going to be white. I'm not sure why you would want to shoot at an ISO like that, even less with a Nikon.
Nikons work best at ISO 400-800 and if you have a tracker, you should keep the ISO that low. The reason why many people shoot at such high ISO's, is because they have to limit their exposure length to let's say 30 seconds, because of field rotation.
Since you would be shooting with a tracker however, you could shoot at the ISO sweet spot of the camera, which gives you a low read noise, high dynamic range image.
Both cameras have a very steep drop off in read noise. The D850 performs well at ISO 400, the D780 at 800.
I think it's safe to say that the difference between these cameras is so marginal, it hardly matters for your purpose. I would go with the cheaper one