The D5300 is a 24Mp camera and the D750 is a 24Mp camera, because fewer of the 24Mp are used on the D750 when a DX lens is attached and all other things being equal, the performance may not be as good on the D750 as it is on the D5300. On my D800, which has a 36Mp sensor I use my old DX lenses because my D800 uses about 17Mp for DX lenses and my old DX lenses were used on a D90 with a 12Mp sensor yielding better performance on the D800. In actuality I only use my 10-24mm DX lens on the D800 because I have FX lenses for focal lengths between 24 and 300mm and I cannot justify the cost of the 14mm FX lens.

You should sell all three lenses and buy replacements. Using a DX lens on a 24MP D750 essentially "throws away" nearly 60% of the image data. Instead of a 24MP FX camera, with DX lenses it will be a 10MP DX camera. Sort of defeats the whole purpose of buying a full frame camera.

It will be a WHOLE LOT more expensive, but FX lenses are what's needed with FX cameras.

The D750 with 24-150mm is a pretty good deal, costing $500 add'l for the lens when it's bought in kit with the camera. Bought separately, the 24-120mm normally costs about $1100. This lens will take the place of your 18-55mm DX. You'll probably get between $50 and $110 selling your 18-55mm, depending upon condition and which version it is.

The AF-S Nikkor 50mm f/1.8 isn't too expensive, at $200. It takes the place of your 35mm f/1.8 DX. You can probably get about $90 to $120 selling the 35mm lens, depending upon condition.

The best way to get approx. 55-300mm equivalent "reach" on a full frame camera is a Nikkor AF-S 200-500mm f/5.6, currently on sale for around $1250. A reasonable alternative that's a little longer is the Tamron 150-600mm "G2" which is selling for about $1300 right now. Both those lenses are considerably bigger and heavier than your 55-300mm. Some alternative for full frame that's closer to the same size and weight, but doesn't have quite as much reach, are the Tamron and Sigma 100-400mm lenses, each of which sells for around $700. I recommend the Tamron because it can optionally be fitted with a tripod mounting ring (sold separately, $129). The Sigma doesn't have that option. You can probably get about $175 to $200 for your 55-300mm lens.

So you're looking at net cost for lenses of at least $1100, in addition to the cost of the camera, if you opt for one of the 100-400mm lenses without a tripod ring. Or, for the longer telephotos, you're looking at around $1600, in addition to the cost of the camera.

The 150-600mm or 200-500mm are also about triple the weight and double the size of your 55-300mm. The 24-150mm is also about triple the weight and at twice the size of your 18-55mm.

The 50mm f/1.8 and 35mm f/1.8 are fairly similar in size and weight.

You'll see better image quality with the 24MP full frame camera... in prints bigger than 13x19mm. For prints smaller than that or images shared online, don't expect to see much difference from what you can do with your 24MP D5300. The other advantage of the full frame camera is that it will probably be usable at higher ISOs, with less digital noise in images. The full frame camera's low light autofocus capabilities might be more of a limitation than image noise at high ISOs. Full frame also has ability to render shallower depth of field effects, but fairly large aperture lenses are needed for that. The 50mm f/1.8 will have some potential, but the other lenses aren't particularly "fast". (Note: Depth of field doesn't actually change with different sensor size... it's only effected by aperture, focal length and relative distances. But when you go from DX to FX, in order to frame a subject the same way you either need to move closer or use a longer focal length, giving the impression of shallower DoF.)

It has been several years since I left the Nikon universe, after ~40 years there, beginning with an F and then a lot of film bodies through the F100 before switching to digital with the D100 and ending up with a D7000 (DX) and D600 (FX). I kept my many FX lenses when I switched to digital, and only "upgraded" to FX during the last few of my Nikon years to get the wide angle benefit of my 17-35/2.8. (My only DX lens was the 18-70/3.5-4.5 - with the better low light performance of the D7000 compared to film it turned out I didn't really need the 2.8 apertures I paid so dearly for in the film days.)

Except for the wider coverage from wide angle lenses, I don't think I saw any real improvement in my images from the FX body. That is, in the couple of years when I used both, I never had the experience going through things in Lightroom to select, process and print the best, of thinking "WOW, that must be an FX image." I paid the money and carried the extra weight for FX in order to get the wider coverage from my widest lens, but I did NOT end up getting "better" quality in the image file, at least not that I noticed.

So I would ask, at the outset, what real-world "improvement" in IQ are you looking for from an FX body that will make the extra expense and the larger size and weight of the FX equipment worth it? And is it possible that you can get that improvement with better DX lenses on your current body? In retrospect I probably could have kept going indefinitely with Nikon DX bodies and lightened my load a bit if I had invested in the (usually) smaller and lighter DX lenses. Instead of dumping it all and moving to Micro Four Thirds, where I am DELIGHTED with the image quality from MUCH smaller and lighter kits of bodies and lenses.

It has been several years since I left the Nikon universe, after ~40 years there, beginning with an F and then a lot of film bodies through the F100 before switching to digital with the D100 and ending up with a D7000 (DX) and D600 (FX). I kept my many FX lenses when I switched to digital, and only "upgraded" to FX during the last few of my Nikon years to get the wide angle benefit of my 17-35/2.8. (My only DX lens was the 18-70/3.5-4.5 - with the better low light performance of the D7000 compared to film it turned out I didn't really need the 2.8 apertures I paid so dearly for in the film days.)

Except for the wider coverage from wide angle lenses, I don't think I saw any real improvement in my images from the FX body. That is, in the couple of years when I used both, I never had the experience going through things in Lightroom to select, process and print the best, of thinking "WOW, that must be an FX image." I paid the money and carried the extra weight for FX in order to get the wider coverage from my widest lens, but I did NOT end up getting "better" quality in the image file, at least not that I noticed.

So I would ask, at the outset, what real-world "improvement" in IQ are you looking for from an FX body that will make the extra expense and the larger size and weight of the FX equipment worth it? And is it possible that you can get that improvement with better DX lenses on your current body? In retrospect I probably could have kept going indefinitely with Nikon DX bodies and lightened my load a bit if I had invested in the (usually) smaller and lighter DX lenses. Instead of dumping it all and moving to Micro Four Thirds, where I am DELIGHTED with the image quality from MUCH smaller and lighter kits of bodies and lenses.