I as you SDK have the 6d currently, but I find it misses focus a lot - maybe I'm just terrible at using it but for me, the upgrade is mainly driven by the superior autofocusing system on the A7III.
I am just a hobbyist photographer, but it really does suck when it looks pretty good on the small screen, then when you get home you realise the focus is out and you've missed your chance.
I'm currently considering whether or not to start again with lenses or just use the adapter, I'll see how it performs when it eventually arrives.
A friend of mine has the 6D original and I own a 7D original. I do not like the limited AF points, no jog, slow focusing on the 6D, and my 7D is very good for taking quick photos, very good AF system . However noisecat 800ISO, limited DR, poor low light performance it does have its draw backs.
Just one big bonus for me on the A7iii is the eye AF and for sure it is a big cheat button. When I hand my 7D to the wife to take a picture of me or another person she hates fiddling around with selecting AF points. So many times she has missed focus on the face. Even with my Sony NEX5R which has a moveable AF point and limited face detection AF she still misses focus. The A7iii eye AF would end all that nonsense for me. Once you are home from a holiday and photos are slightly out of focus. You might as well delete the images as they are useless.
The only down side for me in getting A7iii is the cost of Sony G Master lenses. So expensive !
I know you can use the MC-11 adapter for Canon glass and if I get a A7III I would also get the adapter, and proberly a 24-70 or 24-105 Sony lense, so now a very expensive purchase.
I have watch a couple YouTube videos on camera registration and it appears Canon my be bring out 3-5 New cameras very soon. Though 2 of the 5 could just be different prototypes of the same camera.
The camera world is moving at a rapid pace !
I am not a new camera junkie and my next camera will have to last me at least 5-8 years . So I best choose wisely very soon !