I have both the Tamron 160-600mm f/5-6.3 G2 and Nikon 200-500mm f/5.6. I started birding with the G2 and was getting good results after tuning, but was asking the same questions the op is asking. I eventually picked up the 200-500mm and did a side by side comparison using both a D850 and D500. Both lenses are equally sharp in the center. The G2 is softer in the corners compared to the 200-500 and it gets softer quicker. The G2 is spec'd at 600mm at infinity. For normal shooting distances, say, 20-30yds, it only gets to about 530 to 560mm. It's a long story involving Tamron customer service and repair, but I had to tune my G2 with Tamron's Tap-In to get good results. I do not recommend using a 1.4x with the G2 as it gets to f/5.6 (f/8) around 330mm so AF does not work with my dslrs after that, but I do use a 1.4x with the 200-500 with my dslrs, which makes it an f/8 lens. AF speed is nothing to write home about with either lens on a dslr, but the G2 tends to hunt more. The 200-500 locks and stays locked.
I have used both lenses with my F9. It's a game changer. The G2 performs better on the Z9 than on my dslrs. AF is faster and more accurate and you can use a 1.4x teleconverter, which takes it to an f/8.8 lens, give or take. The G2 still doesn't reach 600mm at reasonable shooting distances and it is softer in the corners, but you shouldn't have to tune it like with phase detect AF on a dslr.
I prefer the 200-500mm, but if you can get a good price for a used 150-600mm G2, it would be hard to pass up. I see them in EX condition for about $800 on KEH.com. I don't see any used 200-500mm f/5.6 on KEH. Maybe that's telling us something.