I am a Nikon guy. I've been shooting Nikon since 2007 I think. so when I got my D500 I was waiting for Nikon to get their act together with the AF of their mirrorless cameras to meet or exceed with D500's AF ability. I got the Z9 (January, 2nd batch) I think you are at this point splitting hairs with the AF system from Sony A1 and Canon R3/R5. The Z9 has a longer learning curve but if you learn it properly, there isn't any shot the A1/R3/R5 will get you the Z9 can't and vice versa. I also knwo the menu system much better AND the Nikon metering system which is not a small thing to me than learning a new system.
The lens lineup is at this point really second to none. Sony may have a few more lenses but the quality of similar lenses I feel are superior at this point. Certainly without a doubt their long glass is far superior at this point (400TC, 600TC, 800PF and 400f4.5). I also can't use a point and shoot size camera like an A1. So for me the size matters. So don't forget that you aren't just buying a new body but buying into a full system which includes the lenses.
That said, for me, if you are considering the A1 at $6500, I would certainly look at the Z9 over the Z8. Battery being one of the main reasons, and the battery life is significantly better with the Z9. The 2CFE car slots are important as the SD slot is mostly useless unless you are shooting landscape/portrait/street/astro type genre, in other words no action/sports/fast wildlife. Then the heat issues even in stills can be frustrating.
I Would seriously consider the Z9 which is still cheaper that the A1 and superior to the Z8 with the best weather sealing, a true flagship body that should much more robust and durable that the A1. I also agree the ergos is a big deciding factor but one can use used to button placement, but as much size and weight. So unless the .9 pounds difference the to the Z9 to the Z8, I'd really look at the Z9 as well.
Just my thoughts and 2 cents