Just to chime in with my story. Not trying to persuade anyone into anything, just sharing my thoughts and hoping to help the OP.
I started my DSLR journey with a great little Pentax K-r. It offered a lot for its' price, more than the equivalent Canikon models. I quickly realized I loved photography and started buying all sorts of lenses...old manual focus Takumars and later on modern DA* lenses as well. In the meantime I also bought a K-30 when it was released.
I also started to do more and more wildlife since it was an area I liked the most. I bought a 70-200/2.8 Tamron which I liked a lot, but it was short as well. I then came to a chance of buying a Sigma 150-500, which I did. I loved the reach, but the IQ was poor on the long end. So I ended up with a DA*300 and a TC
I loved it. Even with its' sluggish AF. When it hit it was gorgeous, but I couldn't really rely on it. So it was either a K-3 or a jump to another brand.
I chose the latter. Ended up selling my quite large collection of Pentax lenses and making the switch to a Nikon D7100.
It was quite a step up in regards to AF-C. True, I'm comparing to a K-30 here whereas the K-3 would probably have been a step up as well, but still...I went at first with the same setup of lenses as on the Pentax. An AF-S 80-200/2.8 Nikon and the AF-S 300/4 Nikon with a TC. When Tamron announced its' first 150-600mm I sold the 300mm and bought the Tamron immediately. I also added a D600 to my setup as well in the mean time. The FX look was in my eyes a lot better than the APS-C when I could fill the frame. But I really hated the small aperture of f6.3. I also needed to stop the Tamron to f8 to get maximum sharpness. Since I was shooting with the D600 most of the time (even though the D7100 offered more reach and better AF), I sold both bodies and bought the D750, which I still own (4 years now).
After having the Tamron for about a year and a half, I sold it and went on to buy a Sigma 120-300/2.8 Sport with both matching TCs. It's a big and heavy lens, but the IQ is phenomenal. With the 1.4x TC attached the IQ barely takes a hit. The 2x TC shows image degradation but is still usable, especially for short distances of up to 20m or so. The AF with the 2x TC takes a hit, but with a D750 it's still good enough to track birds (larger birds) flying head on. What I like about the 120-300 is its' versatility. I can choose to have a somewhat shorter tele but with a large f2.8 aperture or I can add a TC and make it a 240-600/5.6 zoom.
After 4 years of owning a D750 I'm now pondering whether to add a D500 to my setup strictly for long reach situations or saving my money and eventually getting the D850. What I would really like though is a proper D750 replacement...
Again, these are my own thoughts and I completely understand not everyone would feel the same way, but this worked for ME.