The A7 is no speed demon but more than good enough for static people and scenic shots and it'll just about cope with someone at walking speed. That's been my experience anyway. The face detect works quite well, IMO, but I'm not used to the latest kit so the fact that it works at all seems like Voodoo to me. I don't know anything about the D500 but I know a little about MFT as I've had it since the early days. I'd say that an A7 and any of the AF lenses I have (28-70mm, 35mm f2.8, 55mm f1.8 and 85mm f1.8) is possibly not all that bad comparted to the 20mm f1.7 for speed as that lens isn't the quickest. IQ wise an A7 is ahead of the latest MFT 16 and 20mp sensors if you go looking for the differences.
I find all ISO's up to and including the max to be useable with care and basic processing and I've posted lots of high ISO shots here but good luck finding them

If you're really interested I could (somehow) send you some raws to play with or I'm sure you could Google your way to some raws to download.
One thing to think about with the early cameras is sensor reflections with some lenses in some situations but I can't say it's ever bothered me. Other than that I think that an A7 and a compact lens or two could be well worth a serious look if you don't need a speed demon.
PS.
Lens wise you could look at the Samyang 35mm f2.8 and the 28-70mm kit lens.