Just going to throw it out there... but have you tried an E-M1? They can be had for super cheap right now...and the mkII is just around the corner. It is honestly a completely different feel from the E-M5 cameras. Working with it and the focusing aids might be a lot easier. In fact, when set up right, the E-M1 outshines any mirrored camera when it comes to portrait works. The IBIS, SOVF mode, 1/320 flash sync, hi-refresh rate mode combined with the quick magnification from the focus ring make hand held portrait work way too enjoyable. I've shot tack sharp eyelashes at 1/60th ambient, no flash, at the equivalent of 300mm. I can barely handhold my Canon 5DmkII at 105mm and get that sharp of an image.
Yes, I am very biased to the E-M1. My current portrait bag consists of two E-M1 bodies and a Hasselblad H1.
I haven't tried the E-M1, but I am reconsidering M43 as a studio tool so I'll do research on it. The control layout looks not as cramped as it is on E-M5, any notes about that?
I'm well set on the lenses: I have nothing short of magical portrait lens in the 57mm Hexanon, and have a Daguerreotype from Lomography arriving soon, and wide range of great primes already in my bag.
Panasonic GX8 seems to have very good manual focus aids, so that would be another option. Looks like Panny finally managed to put stabilization in the body, so that advantage Olympus had is gone.
E-M1 Mk II is rumored to have dual card slots, which would address one of M43's shortcomings, no matter how unlikely it is for a card to fail (has happened to me).
lowep's post earlier does raise a good point: going from M43 to APS-C is not a big difference, and I know it having gone the other direction once. DXO and DPReview's comparisons confirm this from IQ perspective.
Local Calumet has Sony's cameras on rent, so I'll probably test them out to see what benefit FF offers these days on IQ and usability in the real world.