This is absolute nonsense. In real world use, there is little or no difference between the main brands. If anything, Nikon have a slight edge over the others in terms of lens image quality. But it's really irrelevant tiny details when it comes to actualy taking photos. What you have now is a more than capable camera that should last a good few years more yet. There are countless lenses that fit the F-mount, from enormous long telephotos, to tilt-shift lenses, to oddities like the
UV Nikkor. A better range than any other mount, and better quality than most other brands too.
I use a Nikon D600, so quite an old camera, and it's fine. I bought a Z6 a couple of years ago, and have a few lenses for that now. You can use older F-mount lenses on the Z bodies via the FTZ adapter, and they mostly work fine (you lose AF with some older lenses). The Z6 is a step up in quality over the D600, I have to say. Just that bit sharper (better lenses), and slightly better colour rendition, dynamic range and a significant improvement in low light capability.
Then perhaps consider something from lower down the ML range, IF you did want to go ML. The biggest advantage of the Z6 after image quality, is size. With the 'kit' 24-70 f4 lens, it's much smaller and lighter than any DSLR, so much better for travel etc. The Z50 or ZFC are both even tinier, although they have the reduced APS-C sensor size. The lenses Nikon has released for the Z mount have mostly been excellent, surpassing their F-mount equivalents, as the larger lens mount allows for better optical designs to be used. Sony do have some excellent Zeiss lenses, but they tend to be very expensive. It's swings and roundabouts though really. One lens I really love is the 14-30mm f4. Such a small package, yet optically excellent.
On thing that is good to know about the F-mount though, is that it is so versatile; you can attack a vast array of lenses to it, and equipment such as close up extension tubes and bellows, for proper extreme close up stuff. Your D750 can use many older manual focus lenses, which opens up even more options. I would echo not worrying too much about equipment, and concentrate more on making images. Your D750 still has a lot of mileage in it.