"... The D3 is a beast. So yeah, picking up a D750 after using it, it would feel small and cheap...

Hal, I would advise you to visit your favorite optometrist at your earliest convenience in as I clearly stated I shoot with the Nikon D3x (notice the "X" after the three). Released in December 2008 it remains Nikon's flagship DSLR and I'm likely going to retire while still shooting this epic DSLR (I have a pair and have used them on countless commercial assignments for well over a decade... Note that the D3x has the same resolution of your beloved D750. (I have very few clients who's requirements actually predicate that much resolution).

That said please do follow up with an ophthalmologist (specialist) if your optometrist believes you may have a more complex medical issue... I've had several colleagues suffer catastrophic vision loss due to Age-related Macular Degeneration (a devastating silent condition if not discovered in a timely manner.)

As for your beloved Nikon D750? It still is highly popular even today (October 19, 2023) and flickr statistics validate the aforementioned. See below.

Final thoughts? Hal the most valuable camera accessory I've purchased in the past decade is my flickr Pro Membership. Why? flickr allows you to search via EXIF data embedded within image files.... Thus I'm able to analyze the degree of "commercial publication quality" latent within same. This is possible both by camera body and attached lens. Publication Image Quality (I've discovered) rarely correlates 1:1 directly to popularity... However camera build quality approaches this far more accurately... I assisted many commercial photographers in my market, they all shared the same advice... Wait until the next vendor high end model release and then purchase "Old" new stock at considerable savings. Lesson's learned.

Again wishing you much success on your photographic journey Hal...
Cheers! Thomas

Someone else in this thread pointed out that once sensors reached 24 MPX that they hit a sweet spot of resolution. I have certainly felt that way which is why I shoot my new D850 at 25 MPX. Image quality does not seem to have improved in the last decade, nor has dynamic range. All the improvements are in other areas - frame rates per second, metering, all important AF, and other similar things. So it's not surprising to me that DSLRs of the last decade are still high on the list. New mirrorless cameras are pricey. The old cameras were extremely well built. Why spend money you don't have to spend? If I really needed better AF, instead of just lusting for it ;), or some other aspect of the new cameras, then maybe. But still, you have to learn a new camera and you have to spend a lot of money.

Features are better, especially mirrorless, and then even more: the mirrorless-specific lenses. Changes to the lens mount for mirrorless enable different and better lenses. The newest (and most expensive) mirrorless lenses are amazing wide open, better than any aperture size of the older D/SLR designs.

That said, giving up on great to get greatest is a fool's errand. There will always be something newest and better, but none of those improvements impact the existing equipment. None of the DSLRs turn to dust when a MILC is sold.

The Thomas902 chart would be a great way to investigate the differences, if any. His chart, if you're actively onto Flickr.com, is interactive, in the sense you can click the camera and get a display of images that support that count. What the gear-heads don't understand is you're looking at the artist's edited results, not the native capabilities of the camera body. Those images are more about the lenses, and the digital editing, not the camera or even 'how / how much' the body enabled that result in the hands of the artist.

Features are better, especially mirrorless, and then even more: the mirrorless-specific lenses. Changes to the lens mount for mirrorless enable different and better lenses. The newest (and most expensive) mirrorless lenses are amazing wide open, better than any aperture size of the older D/SLR designs.

That said, giving up on great to get greatest is a fool's errand. There will always be something newest and better, but none of those improvements impact the existing equipment. None of the DSLRs turn to dust when a MILC is sold.

The Thomas902 chart would be a great way to investigate the differences, if any. His chart, if you're actively onto Flickr.com, is interactive, in the sense you can click the camera and get a display of images that support that count. What the gear-heads don't understand is you're looking at the artist's edited results, not the native capabilities of the camera body. Those images are more about the lenses, and the digital editing, not the camera or even 'how / how much' the body enabled that result in the hands of the artist.

Before my local camera repair shop closed in 2020, the owner told me that 70-80% of repairs for camera models with articulating screens were related to those screens. He didn't have numbers for the models with tilting screens, which told me that they do not have anywhere near that high a failure rate.

There are some significant differences between the D7500 and the full-frame D750 and D850 models. Ypu would need to study what these differences are and decide whether they are worth the difference in price to you.