I hear ya on not wanting to sell Canon gear at a big loss. I have an old 24-70II in less than perfect condition that I've been reluctant to sell because I'm not sure I can get much for it. It actually works really well on the a9 and when I got pinned in a small room at an event this weekend an needed 24mm it got the job done. It's much better than adapting on the a7rII.
The 24-70II really will not actively track while burst shooting though. Metabones and sigma need to update their firmwares for the a9. Sony released a firmware for the la-ea3 to make it compatible with the a9's af system and I suspect that in due time sigma and metabones will update their adapters as well.
metabones came out with something today, but I dont have an A9 to test