Tamron 70-210 arrived, but did not work on Z6. Needed FW upgrade - so I had to wait for tap in console to arrive.
Tried it on D600 and didn't work. But then D600 did not work with any other lens - because I did not use it for quite some time, and we had ridiculous humid summer, mirror was stuck. After some careful mirror wiggling everything was back to normal and I compared 70-300 vs 70-210.
At some focal length 70-210 is just a tad better but overall they are pretty comparable. That is not saying that 70-210 but that 70-300 is excellent.
after upgrading FW on the lens I retested on Z6 and got similar results. I compared both zooms, against Nikon-F 105 VR macro and Nikon-F 135 SF and in my opinion both zooms held their own quite well at those focal lengths (where they are the best according to other tests
In any way, I was quite happy with my trusty old 70-300 so today I packed 70-210 and tap in console and returned it back to amazon.
Thank you very very much for your help