Smaller systemYou don't migrate from one full frame system to another to get smaller lenses, not even to DX. If you want smaller lenses you have to go to m43, with more noise. The size gain from going APS-C/DX is too small, imho. Some native Sony FE lenses are smaller, some not. Adapting lenses is easy, no big deal to figure out.
EVFIf Nikon had offered an FX camera with an EVF in 2014, I could have stayed with Nikon. Three years later on there is still no Nikon FX camera with an EVF. I can't properly focus an OVF and have wasted a lot of money on alt focus screens etc. to no avail. FM2 has a much better viewfinder than any DSLR. Go figure.
It might be possible that Nikon could make a DSLR with an OVF to my liking, but so far no cigar, not even close. Should be on their things to fix list, but too late for me now. I have switched to Sony.
Some people switch to Sony to get a full frame system with a viewfinder providing a (10x) focus magnifier. That's what I did. I'm not the only one. And no, liveview doesn't cut it.
Video AFSony and Canon have good video AF, something that Nikon has to fix ASAP. They didn't fix that with the otherwise excellent (from specs) D7500 either.
The next iteration is two years away.
Apps in the cameraSony offers apps for their cameras. The API for that is being reverse engineered, and focus stacking is one useful thing, that has come out of that, though still a proof of concept. Sony could do much more with the in camera apps. Focus stacking is just one small thing that an AF-S or AF-P lens can do. They do that already with CamRanger. Creating an open API for camera apps just one thing Nikon could do, and quickly lead.