Yendor, it sounds like you are either getting very serious about your photography and/or have a bad dose of GAS which most photographers suffer from at various times.
* GAS is an acronym for Gear Acquisition Syndrome, the naming of which is credited to one of our members, LanceB.
I have both DX and FX in Nikon, the DX D7200, and the FX D600. Why do I have both? It's all to do with the size of the sensor and what I will use the different cameras for.
Look here for a side-by-side comparison of the two cameras.
https://www.digicamdb.com/compare/ni...vs-nikon_d600/
As you can see the D600 sensor is 135% larger than that in the D7200, meaning that from the same position, the FOV (Field of View) of the D600 will capture much more of what you see than the D7200 will. This for me is particularly useful for my wide angle shots like nightscapes, landscapes and architecture.
There is a trade-off however. Although both these cameras offer around 24MP, those megapixels are spread over a much bigger area on the FX sensor so in theory will not capture as much fine detail as the same number of megapixels on the smaller DX sensor. At normal viewing sizes the difference may not be very noticeable.
I use my D7200 with it's 1.5 crop factor for detail work, particularly birding, where I strive for as much fine feather detail as possible. The D7200 captures it's 24MP onto a sensor 23.5 x 15.6mm but if I was to crop the D600 shot down to DX size it would only give me 10.6MP.
I hope you see where I'm going with this. Your choice between DX and FX depends on what you intend to use your camera for.
If money was no problem I'd have the new D850 with it's 45.7MP sensor and offering 19.5MP in DX crop mode.
And I totally agree with Tony's comments that a good lens will outlast many camera bodies.
I do however feel you need to move up a notch from the D5500, whether it's a D7200 or a D610, both of which can be had for around $1K.
Decisions, decisions.