Regarding the D750. I would suggest being very careful before you invest in this camera if you plan to do flash work with it. There is a very well known issue with many people reporting that no 3rd party flash works properly (or at all) with this camera because of a hotshoe issue. There are several other issue with this camera as well, with this is one that I still see complaints about.
The advantage of having a small and light camera is that it can be small and light in some situations, while the big heavy camera will always be big and heavy. It is the element of choice that is missing in the larger cameras. You can make a small and light camera large and heavy by adding a grip, but what can you do to the large and heavy camera that will make it more compact? There are other advantages to mirrorless cameras, such as a lot more information and analytics in the viewfinder itself, and removable (which means changeable) viewfinders. We may not see these utilised much currently, but it is only a matter of time that these start making a difference to photography.
Are you really OK with using your existing lenses on crop mode with a full frame?
I haven't followed the MILC vs. DSLR battle in great detail, but whenever I see MILCs being used it is largely for portrait/glamour/weddings/landscape. I admit these are very popular and profitable, but these aren't torture tests of autofocus performance. Can Sony's on-chip phase detect autofocus match, or beat, the dedicated phase detect system in your D7000, or a D500, in demanding applications like sports, and particularly for birds-in-flight photography which is very important for you? Do you know any Sony photographers with whom you can do a side-by-side comparison of capturing birds-in-flight with same time, same location, same light, same cranky bird? It will be interesting to see how that pans out. Remember, in a Sony camera the parts of the sensor that are dedicated to phase detect do not capture image information (unlike Canon's DPAF). There are some demosaicing algorithms that you cannot use with a Sony camera because they reveal the dead pixels. Sony's cameras have also been criticised by astrophotographers for their 'star-eating' algorithms that destroy a lot of detail even in the raw file, so everything isn't perfect on the Sony side.
Since you don't have a pressing need to upgrade your camera, why not wait to see what Nikon actually does in the mirrorless field, and how well it does it, before thinking of a switch? We should have a lot more information by the end of this month when the new body with the Z mount is finally unveiled.
i am very interested to upgrade my d7000...saw some great used offer on d750, d800+ 24-70, a7r, 5dmk3. Etc...