Camera wise, the Z8 rips thru battery charges, quickly. I used a Z8 for a few months, then dumped it for another Z9. The Z9 battery life will be surprising to you. The red signal will come on and still load both cards with another 125-150 photos.
My comment for lenses, you have 3 zoom 2.8 S Nikons, 14-24, 24-70 and 70-200 - should be perfect for the basketball. Adding more glass, in my opinion, should be investment in Z lenses, (not G or E).
Regarding battery life on the Z8….I simply do not see this issue whatsoever. I have owned the Z8 for a little over 18 months and there has not been a single instance when I’ve had to use a second battery during a morning/evening outing. My outings are usually 2-4 hours in the morning and I have taken up to 2,000-3,000 images during this time on a single battery. Since the DSLR days I’ve trained myself to keep the camera OFF if I do not have my eye up to the viewfinder. This prolongs battery life exponentially.
(Disclaimer: I only shoot stills, so if you’re shooting video this may be an entirely different conversation)