Currently i am using the 300pf with all three converters on a Z6 II. Sold most of my gear and waiting for the Z9 specs and the rumored Z AF firmware. After its release i will decide which system to choose.
The 300PF works well on the mirrorless cameras and the image quality is relay good.
With DSLRS its a different story. The TC 17 II is usable and delivers good quality but the TC20 III is more of a last resort and i seldom used it with the D500 and D850.
You definitely have to fine tune the TC´s with DSLR´s but after fine tuning they will yield good results.
If you have enough light you should be able to do some action work with the D500 too. In this case the Z6 is lacking so fingers crossed for the rumored AF update or Z9.
And now some images with the Z6 II + TC 20 III. Spent 3 hours with a pair of ringed plovers. Those are the moments i enjoy most. They let me move close to them. Crawled slowly within close focus distance. That experience included some blue marks as the stony beach wasn't comfortable but that's a given fact in wildlife photography
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Nikon Z6II with 300 PF + TC 20III | ISO 250 | 600mm| F9 | 1/320s
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Nikon Z6II with 300 PF + TC 20III | ISO 250| 600mm | F9 | 1/400s
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Nikon Z6II with 300 PF + TC 20III | ISO 280| 600mm | F8 | 1/800s
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Nikon Z6II with 300 PF + TC 20III | ISO 250| 600mm | F9 | 1/400s
*edit
I will try to get some images this weekend with the TC17II