These two list above points to the full frame 6D for sure.
- Night Sky Photography
- Head Shots
In fact, all of them point to the 6D being the better choice.
I guess my problem is that in light of the 7DmII and the 5DmIII I feel like I am getting robbed on features and the AF system just seems substandard at this level. Of course the price point for the 6D is right.
That does seem to be Canon's strategy. The 6D delivers excellent IQ particularly in low light, the 7DII delivers top AF and speed. If you need both, that's the 5DIII...and it actually costs a bit less than the other two combined.
I do have 4 and 6 year olds that will be getting into sports and ballet which a 5DmIII would do well but I think the 6D would be questionable.
Low light action is best served by excellent AF and a FF sensor...and fast lenses. None of that is cheap. In gymnasium lighting, I often need ISO 6400 or 12800 to get a 1/640 or 1/800 s shutter speed to freeze motion...and that's with an f/2.8 lens wide open.
With the exception of your kids activities, the 6D is the ideal choice. Although the 6D's AF isn't up to the level of the 5DIII or 7DII, good technique can help there.
I really think a FF body is a better option for your needs; but...you initially asked about lenses, and they're important too.
Important lens characteristics include:
- Night Sky Photography - wide angle, fast aperture (but for the moon you want the longest lens you can get, and it can be slow)
- Head Shots - short/medium telephoto, fast aperture
- Weddings - standard zoom, f/2.8 preferred; telephoto zoom, f/2.8 preferred
- Maternity - standard zoom, f/2.8 preferred; short/medium telephoto
- Couples - standard zoom; telephoto zoom (neither need be fast, you'll be stopped down to at least f/4 for sufficient DoF
To have the optimal lenses for a diverse array of subjects means several lenses (which is why I have 5 zooms and 9 primes!). The aperture suggestions above are based on ambient lighting; if you're going to set up a home/portable studio (backdrop + stand, flashes or monolights + modifiers), you'll be stopped down a fair bit and slow lenses are fine. Don't forget that some of the non-L primes deliver excellent IQ, and with session work (headshots, maternity, couples) you'll have time to move so the flexibility of a zoom isn't critical. The 85/1.8 is one of the best values in terms of IQ/$. The Samyang 14/2.8 would likely work well for night skies.
Given your budget and needs, do consider refurbs (assuming you're patient but quick, desirable refurbs come up periodically and sell out fast). A refurb 5DIII is $2700, a refurb 24-70/2.8 II is $1700. You could start with f/4 zooms, and move to f/2.8 versions later.