You'll need to replace at least three of your lenses, if you get an FX camera. Your 18-140, 17-30 and 11-16mm are DX lenses. Designed for use on DX cameras like D7100 and D7200. Probably your 50mm and 75-300mm are FX lenses that would be usable on D750 FX camera, as would be the 300mm and teleconverter you are planning to buy.

Yes, you can USE the DX lenses on the FX camera. HOWEVER, when you do that the camera will automatically crop images to DX size. With a 24MP D750, what's left after cropping is under 10MP. You currently are shooting 24MP DX size images with your D7100, so there would be a big drop in image quality using DX lenses on that FX camera, versus what you have now.

So it's really pretty much a waste of money to buy an FX cameras and then just use DX lenses on it. You're better off using a DX lens on a DX camera, in all but the most extreme cases (such as 46MP D850). In other words, if you "upgrade" to an FX camera, you also need to plan to upgrade to FX lenses, to make that camera upgrade worthwhile. And you should anticipate needing bigger, heavier, more expensive lenses, too... For example, instead of the 300mm f/4 lens and teleconverter you're planning to buy to use on your D7100... in order to have the same "telephoto reach" and speed with any FX camera, you're going to need to get a 500mm f/4 and teleconverter instead. Compare for yourself

By the way, have you considered the Nikkor 200-500mm f/5.6 zoom instead of the 300mm + 1.4X?

There aren't a whole lot of differences between D7100 and D7200. The newer model has bigger buffer, slightly improved AF system (low light, tracking movement), and slightly improved 24MP sensor (slightly wider DR, a bit higher ISO capable). Depending upon what you shoot and how you shoot it, you may not see any difference.

http://cameradecision.com/compare/Nikon-D7100-vs-Nikon-D7200
https://www.imaging-resource.com/cameras/nikon/d7100/vs/nikon/d7200/
https://www.dpreview.com/forums/thread/3977340

Yes, the D7200 has built in WiFi. I would recommend you investigate what people actually using it think about that feature. Typically there's pretty limited range.

If the only complaint about your D7100 is ability to transfer images wirelessly, maybe get a Camranger to use with the camera instead.... problem solved. A Camranger has about 3X the range of standard WiFi, as well as much greater reliability. While far better than built in WiFi or WiFi adapters, Camreanger's 150 foot range is not as much range as Wireless File Transfer modules available from Nikon themselves, but those are a whole lot more expensive than Camranger (and I don't know if Nikon even makes one for D7100... this is NOT the same as the WiFi adapter you tried... It's a much more powerful wireless network adapter).

BTW, if you are sending the images to your wife's phone or tablet computer, the memory cards of those will fill up FAST, when 24MP images are sent to them.

P.S. While it really makes sense to buy FX lenses for FX cameras.... But it doesn't really make sense to only buy FX lenses for use on DX cameras. I've been using full frame and crop sensor DSLRs side by side for years. While a lot of my lenses are full frame capable and usable on both, I also have some crop-only lenses. Those offer smaller size, lighter weight and lower cost... and is some cases even better performance than similar full frame cameras. Mostly wide angle to short telephoto/macro/portrait.... crop only lenses can be advantageous. Anyone who has a crop camera, but refuses to consider and use crop lenses and insists on only buying full frame capable lenses, is just short-changing themselves. They're probalby spending more money, carrying around more weight than necessary... and will be unable to do some things.

interesting what you said about transfering 24MB images to the wifes phone. In DX mode these would be 10MB images instead. Some people argue that a 10MB image is of lower quality than a 24MB one but the difference is microscopic. There is a limitation to the resolution of the human eye and 10MB is more than enough for an 8x10 at 25cm viewing distance. For a screen of 1920x1080 there are just over 2 Mega pixels. Unless you are viewing at 100% crop can you tell the difference in resolution in detail that normally contributes a fraction of a pixel?

10MB images may actually be a positive in this case.

interesting what you said about transfering 24MB images to the wifes phone. In DX mode these would be 10MB images instead. Some people argue that a 10MB image is of lower quality than a 24MB one but the difference is microscopic. There is a limitation to the resolution of the human eye and 10MB is more than enough for an 8x10 at 25cm viewing distance. For a screen of 1920x1080 there are just over 2 Mega pixels. Unless you are viewing at 100% crop can you tell the difference in resolution in detail that normally contributes a fraction of a pixel?

10MB images may actually be a positive in this case.