What is important in your field of photography is that the pictures please to the one(s) who look at them.
To get a good picture, the photographer’s eye and abilities are far more important than the tools (s)he uses.
Why am I stating this? Because apparently you succeeded taking pleasing pictures and have clients who recommended you to others.
However - at some moment in time - the technical specifications of the tool become a limitation to what the photographer wants to achieve and (s)he is therefore looking out for new tools. This is where you are now.
In the technical specifications upgrade path that you consider there are three aspects:
1/ ergonomics that come (on top of technical specifications) with the camera line (the D3xxx/D5xxx, the D7xxx/D7xx, the D500/D8xx)
2/ full frame FX versus DX
3/ lenses
Aspects to consider here, related to your current camera and lenses, are:
- as you keep your D3500 as a backup, using a different « line » will bring you new ergonomics. While these ergonomics offer extra comfort, you will have harder if you use the two cameras together. As there are number of advantages with improved ergonomics, this should not be a hindering factor in your decision, still one to consider.
- some of your lenses may be DX marked. Based on the specifications you gave, I could not identify which ones, therefore have a look here to find out:
https://www.nikonimg...00005532〈=en_GB
Also, you have to consider that there is a difference between AF-S vs AF-P full lens compatibility with some older cameras
https://www.nikonimg...00035705〈=en_GB
Does this mean, that you *have* to replace your (DX) lenses when you (possibly) move to FX? No.
Just consider when using a DX lens on an FX camera the cropping factor will have (mostly) one effect (when your FX camera is set to DX crop): the picture projected on the sensor will not cover the full area of the sensor and therefore not use all pixels. For this configuration the resolution of your picture will have less than half the Megapixels that the sensor offers.
Is this an issue? All depends how much you want to additionally crop (in post processing) and how large you want to print.
But as you will keep your D3500, you will be able to continue using those DX lenses as you are used to with that backup camera.
On the other hand using an FX lens on an FX body, while having used it so long on a DX body, will have an « uncrop » factor, as already mentioned by Peter.
So, what is the best path to upgrade from your current equipment?
It partly depends on how much of this upgrade is driven by business needs and how much is to improve on photoghraphy in general, as well as what your budget is.
Moving to a D750 will possibly give you the best cost/benefit ratio, as it allows you to have the advantages of an FX sensor at a reasonable price, provided that some of your lenses are designed for FX; but possibly also check the follow-up model (D780) if you also have already AF-P FX lenses.
As for business, consider additionally that having an FX camerabody is a vending argument to attract new clients. Independent of the quality of the photographer - as mentioned in the introduction - number of paying clients still look at the tools to evaluate how serious you take your business.