Hi Gil, I only have the 300mm PF but have been really pleased with it
Both performance and portability
The 500mm PF is available to hire from many places
I don't think there's any substitute for actually using a lens.
I'm in the UK and have found quite reasonable rates for (say) a 3 day weekend
That gives you 3 separate chances to try it out under different conditions, plus you can get used to its size, weight and handling.
Plus review images from the 1st two days whilst you still have the lens
Many years ago whilst I still had an old 300mm f4 AFD, I rented the then current 300mm f2.8 behemoth.
That was a real eye opener.
In my case, I struggled to match the results I'd managed of large hawks flying at close range that I'd previously got with the smaller lighter version. I just didn't have the strength to pan and hold it for more than half an hour at a time.
The pic below was taken with the older f4 version

I never had the budget to actually buy the better lens, but renting really helped me focus on real world use.
In your case, you could also use this approach to try Akira's suggestion of a Z body and suitable Z lens