I would summarize the biggest advancements in sensor design and processor tech since the D750 was introduced 11 years ago as follows:
- Increased resultion
- Increased data processing speed
- Improved autofocus
- Improved video
The D750 is a 24MP DSLR body with excellent dynamic range, good autofocus and a middling burst rate & buffer. The Z6III is the modern day equivalent. It has the same 24MP resolution but superior autofocus, burst rate, and buffer. The Z6III was introduced in June 2024 and hasn't been plagued by the quality control issues that affected the D600 and D750.
Nikon's professional mirrorless bodies are both built around 45MP sensors, and feature better autofocus & buffers, superior build quality, and video performance. When it comes to video, there's no DSLR that comes close to matching the video performance of today's mirrorless bodies.
If you combine these advances with AI noise reduction and sharpening tools, today's best mirrorless bodies are routinely able to output publishable 45MP+ images at ISO 12800 & higher.
Of course, one thing hasn't changed in the last decade-plus. It's the person using the camera that's the biggest factor determining image quality. Any Nikon digital camera from the last 15+ years, if in the hands of a skilled photographer, can be used to make publishable images.
Whichever mirrorless body you choose, you may not find an immediate, obvious jump in image quality in comparison with your D750 images. But depending on how you shoot, you'll find yourself coming home with 3 to 5 times as many keepers. You'll also be able to make keeper images in lighting conditions you wouldn't consider a possibility with the D750's limited AF system.
Also, be prepared to rethink your data workflow and storage solutions.
Good luck with your choice of new cameras.