In my opinion, the best tool to do such a comparison is DpReview.com. Here is a link for the tool, you need to invest some time to find similar cameras.
https://www.dpreview.com/reviews/nikon-d7500-review-speed-and-capability/8
If you want to see the specs, use this other comparison page https://www.dpreview.com/products/compare/side-by-side?products=nikon_d700&products=nikon_d7000
https://www.dpreview.com/products/compare/cameras
The problem is that the D70 is too old for these tests. But you probably can dig in to find the old comparison test page.
For your specific case, you need to take into account if you really need or want to go for a larger sensor, because any DX lens you have will not be usable on an FX format.
If you only have one DX lens, there is not much to lose there.
But I would not go for an oldie full frame camera, in my opinion, a newer DX camera will have better features. You probably do not think you want or need video, or better battery performance or more megapixels.
I love Nikon, my first camera was an FM2. But as you seem not having a lot of already purchased Nikon gear (lenses) you might consider other brands to suit your specific needs. Canon, Sony, Fuji?
Reading the comments you posted, probably the real issue is low light performance. Remember that you can go for a faster lens, a noise reduction software. But in any case, still, a newer sensor will perform better in low light than an old one.
In some other cases, having a 24 Mpx image with good enough low noise will be a heck more smooth if you resample to your mentioned 6Mpx. This way, with a 24Mpx sensor you can push your maximum ISO you are normally willing to accept.