Greetings,
I've got the Sigma 150-600c
I do not care for Tamron lenses because the zoom rings operate in reverse of Canon and Sigma glass. Maybe this won't bother you but I do not care for it. All are long-throw zooms. Going all the way in or out takes two grip and roll rotations.
Many praise the Tamron for sharpness and updated lens coatings, compared to the Sigma's Contemporary and Sports lines. I never noticed a dramatic difference that made me feel I had made a bad choice. The Sigma Sports model has higher quality optics in comparison to the Contemporary version. You'll pay for it both in price and weight.
Biggest differentiator for me. Historically Tamron doesn't do as good of a job updating the firmware on its lenses. Tamron lenses have more compatibility issues with newer bodies than Sigma ever has. Sigma has also been much more responsive addressing these types of issues. Regardless of which you buy, you'll want to get the Sigma Dock or Tap-In to upgrade and adjust (if needed) either lens.
We know the 150-600c does not play well with the R7. All of these lenses perform about the same on MILC bodies. They do not have the same AF capability that Canon EF or RF glass has. Some days, scenarios will be better than others. Lighting, background and subject. Dark bird, dark background the AF might struggle a bit. Reeds, and branches can also cause some hunting and the AF may lose tracking while panning, especially if the background is busy. Stationary subjects will be better. Other items which affect AF with these lenses. One Shot vs Servo AF, IS (which you pretty much need for handheld shots) and distance to your subject.
Do not take what I've said as gospel. I am not a wildlife shooter. I primarily shoot architecture and landscapes. I have shot enough with my Sigma lenses on both DSLR and MILC under various conditions to see the strengths and weaknesses of each. What do I ultimately recommend? You will have the best experience with Canon glass, whether adapted or native. 3rd party EF lenses do not focus as quickly or track as well as Canon glass does. Its very close in some cases but Canon glass will deliver a higher number of keepers in the end.