The biggest advantage to a prime lens is that it's (usually) going to be faster than a superzoom telephoto like the Tamron and Sigma 150-600s. I use the Tamron G2 150-600 with my Canon 80D and R7, both of which are crop sensors, and get great results. On a full frame like the Z6 the lens and resulting images would look even better (without the extra reach of the crop factor). That said, there's no replacement for the light gathering and silky bokeh and background blur that you'll get from something like a 600/f4 or 400/f2.8 vs the 5.0-6.3 aperture on the Tamron. On the high end you've got about a stop and a half extra light available on the 600/f4 vs the 150-600mm zoomed in and at f6.3. The quality of the glass in a prime telephoto is going to be way higher than a superzoom telephoto and you have fewer moving parts to introduce issues with alignment, focus, sharpness, degradation in quality, etc.
They're just better tools for the job if you are shooting wildlife, sports or other subjects where you need the reach and can't sacrifice quality. New, though, they cost about what a decent used car does often and that's just not attainable for amateurs or even semi-professionals (used will be a better deal and often you can find examples that are in great condition for a fraction of new pricing).
I do disagree with the comment that you should upgrade your body, though. A fairly recent full frame mirrorless camera is not going to suffer from any sort of quality concerns using the Tamron. The Z6 is two years newer than my 80D and a full frame body, so you should be able to get great images form that combo. Those pursuing photography as a serious hobby or career are better suited purchasing quality lenses and mediocre bodies than the other way around.
Yes, there will be some noise and you can only crop so far unless you are shooting some absurd 40mp+ full frame body but software like Topaz DeNoiseAI and others can really perform magic on grainy images. That said, they are only as good as the image you start with - some can be just too far gone.
Tamron G2 150-600 80D and R7 Examples:
Short-Eared Owl by
charliebravo77, on Flickr
CO/SD/WY 2019 by
charliebravo77, on Flickr
The Tamron G2 70-200/f2.8 is probably my favorite lens to shoot wildlife with if I can get close enough, due to the fast aperture, low light abilities and tack sharpness. Examples:
Suffice it to say that unless you are shooting photos for profit the Z6 and Tamron is an excellent combo and if you don't already have one, the Tamron 70-200/f2.8 would be a great addition.