@JJ
Have had a good look at the images you took recently with your new A9 and compared them to some similar images you've taken with the A73 and also with the Nikon D750.
Overall, I think you're making far too early a decision to ditch the A9.
There's so many factors that can lead to the poorer images you've seen from your first trials with the A9. The main ones as I see them are as follows:
1) Unfamiliarity with the A9's AF system. This camera is apparently better / different compared to the Sony A73. The way it tracks and sticks with an acquired target or the way it is attracted to and switches to a new target is probably going to be quite different to every other camera you've tried and will take a lot of getting used to in order to get acceptable results.
2) I don't know too much about the Sony auto focus modes and can't really be bothered to research either but it may be that the modes you've tried are not the best suited to the images you are attempting to capture. I'm a Nikon guy myself and the Nikon AF modes really do take some getting used to and there have been some changes to the way in which these modes work on the newer D5, D500, D850 models compared with the older models. I am still getting used to them on the D850. The performance isn't just different it also varies with lighting conditions, with exposure settings and the subject that you're trying to shoot. The way in which the subject is moving and whether the object it is in the clear or partially obscured and what else is in the same image are all factors that need to be experimented with and tried with different techniques.
3) The weight and balance of the A9 with the Sony 70-200mm lens. Again, its different to what you've shot with before and will take some getting used to in terms of the way to support the camera and lens to avoid camera shake when tracking, especially at low shutter speeds. Practice, is the best way to improve technique. You've had plenty of practice with the Nikon D750 and obviously your handling technique has been perfected with that camera / lens combination. You really haven't had sufficient time with the A73 and certainly not with the A9 to even come close to the levels of expertise you were achieving with the D750.
Hope you stick with it and give it a really good trial. In the meantime I'm off to create some space for you on the Nikon D850 thread. Maybe see you there soon !!!!