If you want to stay relatively inexpensive, you could try a used Tamron 150-600mm g2. I'm still shooting a d500 so I can't comment on how it works with a z8 and FTZ. But you could probably get one used in excellent condition for not much more than the cost of fixing your 200-500. You'd have an extra 100mm. But you will lose a half stop of light. It's f6.3 at 600mm.
I'm another person who was taking photos mostly on the long end of my Tamron 150-600mm's range, so I got a 500PF and love it. I do sometimes miss the 600mm range. But I plan to use my 500PF with a z8 when I eventually switch. Many people on this forum report good results.
I'm another person who was taking photos mostly on the long end of my Tamron 150-600mm's range, so I got a 500PF and love it. I do sometimes miss the 600mm range. But I plan to use my 500PF with a z8 when I eventually switch. Many people on this forum report good results.