Hi and welcome!
Well, I am a retired (so now non) professional wildlife photographer, although I will make sure my comments are not unprofessional! 😊
I owned the Sigma Contemporary version, and used the Sports version of this lens, both with DSLRs and the R-series MILCs. TBH, there is not much difference optically between the sports and contemporary versions - the main differences are in build where the sports is much heavier but with weather sealing and stronger materials - and that is not surprising, as it's built for the rigors of professional use. I would be happy to use the C version for work as I baby my gear and don't subject it to dust and moisture without protection.
I shot for some time with both FF and APS-C DSLRs and the Sigma (C from now on) behaved beautifully, and I found the same thing with the FF R-series bodies: R5 and R6 MILC variants that I use, via the Canon EF-RF adapter. I have not used it for landscape, especially in the confines of the city - I would tend to go for wide angle lenses to get the FoV, but that is your creative decision. While I have not used the EOS R body, I would expect that is should be much the same. The behaviour of the lens IS and optical quality are just as good.
There have been issues raised about focus pulsing, and back or front focus with the lens used on the EOS R7, but it has been pretty much established that this is a function of the R7 and not the lens - and, in fact, those issues were not limited to this lens.
I don't live in the US, but for a used lens I referred to KEH.com website and they come in at about $800 and have a warranty, FWIW.
The other lens I would consider, but it will be more expensive, is the Sigma 60-600 Sports. I had it along with the 150-600c and sold the latter because the 60-600 has a much greater focal range and, while more expensive and heavier, it is a brilliant optic and is a much more all-in-one solution. I have used that on both DSLRs and R-series MILCs.
I bought a Sigma 150-600 Contemporary for use as a decent general lens for hiking when I didn't want to take my larger and heavier Canon glass. I was very surprised at the results from it, the first four images below are from the day I bought it and gave it a quick test.
All of the images were captured using this lens with a 1DX III body and none were at low ISO, the lowest was at ISO 1,000 and the last hummingbird was at ISO 10,000 so even with DPP doing its usual processing to reduce the impact of noise the lens still provided good sharpness and color. I have ended up using this lens a lot more than I expected and I have been very happy with it.
For HS sports, available light may create some issues but that will be true of any lens with the aperture range of these "Super zoom" lenses. I use a Canon EF 400 f2.8 for a lot of high school sports but it weighs a lot more and costs 12 times as much making the Sigma an excellent value when conditions are within its shooting capabilities.
The Sigma 150-600 has been on a lot of hiking trips with me and has always worked well. The last photo is from my first hiking trip with it on a typically cloudy dark day around Cosby in the Smoky Mountains forcing the 1DX III to ISO 32,000 for a standard exposure.
Rodger
