It's a hard choice. I hope you are able to find an option that works well for you.
I really like my EM5ii. I have a couple really good pieces of glass and I have faith that the M4/3 system is only going to get better and better.
Like any camera, it has its weaknesses. The major weakness I find is focus tracking of moving objects (birds in particular). For all else, I've found it to be superior to my old D5100.
Some features of mirrorless cameras are Really cool, like looking through the EVF and seeing EXACTLY what the picture is going to look like at the snap.
Also mirrorless cameras have some nice long-exposure settings that eliminate over-under exposures. Again, VERY cool.
You say you're concerned with dropping to that small of a sensor. Why is this? Depth of field sacrifices? Low light capabilities? I would recommend researching professional photographers that use M4/3 system for their work. There are definitely some fantastic lens options to support this line of work, you just have to know the limitations of the camera (which also goes with any other camera as well). I've done a couple portrait sessions with this system, and they've turned out pretty good!
You can't go wrong with the Olympus 12-40 f2.8. It's a fantastic lens at a great price. You can find one used for less than $800. The prime lenses are also fantastic. I have the Olympus 45 1.8 and Panasonic 25 1.7.
Best of all with my system, it's SMALL.
It sounds like this is the reason you're selling all your other equipment, because it's cumbersome and heavy to lug around.
This is the exact reason I went with M4/3, for the size. It didn't make much sense for me to buy a Sony or Fuji and still have a fairly large system.
Of course, this works well for me, and Not necessarily another shooter. Hope this helps. Good luck in your search.
Oh, I already know a photographer that has been using a M4/3 camera for professional work for a while now.
Sergei uses an E-M5 and does some amazeballs work.
Featured: 'Mu43 Wedding Experience' by SSSYURRR
(I used to haunt this site back when I had my Oly E-PL5)
anyway...my biggest concern with the smaller sensor was low light performance. Granted, maybe the E-PL5 was just not a very good camera to base the M4/3 system on, but it really sucked in low light. also, the 3 axis in-body stabilization also kinda sucked. The sensor is just so much smaller than even a DX sensor, and there is just no way to really get around that fact.
I think at this point, its more of a mental issue. A not-so-great experience with a M4/3 camera coupled with not wanting to give up the advantages of a full frame sensor that I have come to enjoy.
The real facts of the case are this however...I really don't think whatever camera I get is going to be used all that often.
on one hand, I could get into an Olympus system comparatively cheap, and it could sit in the camera bag just as easily as the Sony a7. on the other hand, on those rare times i actually feel like dragging the camera out, i wouldn't mind having a nice camera.
what IS appealing about the m4/3 system is that it is small, and I can still fulfill my requirement of having a viewfinder and hot shoe flash. up until now, I would grab the Nikon1 when we go out instead of the D600 just because of how much a pain-in-the-ass hauling that big camera around was. Im really trying to talk myself into an Oly setup just because I think its a camera I might actually take out and use.