The only thing you want to do is keep it clean using q-tips with rubbing alcohol. Your ballhead might just have some residue on it. Watch this video.
https://www.youtube.com/watch?v=eR_a8I3Zd_I

Since a 400mm focal length is sufficient, a cost effective solution is to get the Tamron 100-400mm for your D500. It weighs 1115 grams as opposed to the 2300 grams for the Nikon 200-500mm. I have rented it before. It's an excellent lens, especially considering that it's only $800. I got great results with it at an airshow. Continuous AF tracking worked great with the lens. I already have the Nikon 80-400mm so I didn't purchase one..

Here are the specs on the D500's autofocus points.

All 153 autofocus points are compatible with f/5.6 or faster lenses
The center 15 points work with f/8 lenses
99 are cross-type

All of the focusing points are available. I think what you may be referring to is the sensitivity of the autofocus points. The center nine points are rated to work reliably I think with an f/8 aperture. The rest are in the f/5.6 to f/8 range. There are 99 cross type autofocus points. I'm not totally sure of the exact count though. More research could be done on this.

Remember to take some neutral density filters. Without a neutral density filter, you may have trouble obtaining a slow enough shutter speed to smooth out the waterfalls. You might end up stopping the aperture way down to slow the shutter speed. Stay away from variable density ND filters. Just bring a couple of fixed ND filters. Something like a four stop and six stop. Other UHH members here may have suggestions on ND filters also.

If you have any DX lenses, I would trade in the D90 and D600 on a D7200 or D7500. They have a higher pixel density and will give you the extra reach. Keep the D800 also. I have a D7500, D500, and D750. I like having both cropped sensor and full frame.

I have two of the Paul C. Buff DigiBee DB800 strobes. The nice thing about this model is that it has digital control. With theses strobes, I can use the Paul C. Buff CyberSense controller. I can control up to four strobes. The power level and modeling light output can be controlled simply by sliding your finger on the face of the CyberSense. Only the Paul C. Buff DigiBee and Einstein strobes can utilize it.

I also have Paul C. Buff strobes (the DigiBee DB800). Its optical eye can sense a flash and trigger the strobe.

Godox and Flashpoint are the same. Adorama Camera's Flashpoint series are rebranded Godox equipment. It may work.

Flashpoint is Adorama Camera's brand of equipment. You might want to give Adorama a call to make sure that the receiver is compatible with your transmitter.

If your Nissin flash is for Nikon, you could use this receiver. Using a Nissin flash made for Nikon cameras is the same as using an actual Nikon flash.

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I'm pretty sure this Flashpoint receiver will work with your Flashpoint R2 promark II transmitter (Nikon compatible model). You would mount your SB-900 on top. If your Neewer speedlight is for Nikon, it should work with it also. You could call or email Adorama Camera to make sure of the compatibility.

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Which Flashpoint transmitter do you have?

You could put the SB-900 on a Flashpoint receiver. If the receiver supports TTL, you would leave the SB-900 in TTL mode. If the receiver is a trigger only unit (non-TTL), you would put the SB-900 in manual mode.

The SB-900 has a photoelectric eye. It's normally used with Nikon's CLS (creative lighting system). It's possible that with the SB-900 in manual mode, the flash from the Flashpoint strobe will trigger it. Try that out.

I have used my SB-700 flash with a Paul C. Buff DigiBee strobe in a studio. I had the SB-700 mounted on a PocketWizard Flex-TT5 transceiver. I had a PocketWizard Plus III receiver connected to the studio strobe. The PocketWizard Mini-TT1 transceiver on my camera triggered both of them.

Definitely get an autofocus lens and try continuous AF tracking (AF-C). I was asked to help photograph a runway fashion show once. I put my camera in AF-C mode for continuous tracking as the models walked towards me. Here are some of the photos I took there. The lens was a 16-85mm f/3.5-5.6 set at f/5.6

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