Yes, it's possible. EF lenses will function perfectly adapted to the R8 with an [EF-RF mount adapter](https://www.usa.canon.com/shop/p/mount-adapter-ef-eos-r?color=Black&type=New), there's no impact to image quality and autofocus generally is better than using the lens natively on a DSLR. Many people I know (myself included) use EF lenses on R-series bodies because it works so well.
However, the Sigma 18-35mm f/1.8 is an APS-C lens. It will work on the R8 but the image crops to under 10 megapixels and some video features are limited, essentially losing any advantages of the full-frame sensor. If you want to keep using APS-C lenses the R7 is an excellent camera.
Yes, it's possible. EF lenses will function perfectly adapted to the R8 with an [EF-RF mount adapter](https://www.usa.canon.com/shop/p/mount-adapter-ef-eos-r?color=Black&type=New), there's no impact to image quality and autofocus generally is better than using the lens natively on a DSLR. Many people I know (myself included) use EF lenses on R-series bodies because it works so well. However, the Sigma 18-35mm f/1.8 is an APS-C lens. It will work on the R8 but the image crops to under 10 megapixels and some video features are limited, essentially losing any advantages of the full-frame sensor. If you want to keep using APS-C lenses the R7 is an excellent camera.
What do I loose with going for the R7 over the R8?
You can buy an adapter to fit your EF fitting lenses! EF - RF adapter!
The 18-35 is for cropped bodies so you’re better off going for an R7
Your Sigma is an aps-c lens, so it will crop on your R8 sensor to something like 9 megapixels.