In the meantime, I have bought a second third-party lens:
The Rokinon (Samyang) AF 14mm f/2.8*.
I enjoyed using it with the Sony Alpha 7R III before. But since my Sony had to leave in the meantime in favour of the EOS R5, I also sold the Samyang AF 14mm f/2.8 FE.
The lens is a real insider tip! The image quality is unbelievably good for the price, it even comes close to the image quality of the Canon EF 14mm f/2.8L II (see the direct comparison on the-digital-picture.com), even surpasses it partially - and that for less than a quarter of the price.
I now had the choice to choose either the EF or the RF version, the lens is produced with both mounts. But since I want to use it on my 5DS R as well and found an extremely good deal, it finally ended up being the EF version.
The lens works fine with the Canon EF-R mount adapter on the R5. The eye autofocus is also working perfectly. Our house cat Tom had to serve as test object:
The autofocus area is only slightly limited (about 80% each horizontally and vertically).
However, there is another limitation with the samyang:
For reasons I can’t explain, on the EOS R5 with the Samyang 14mm AF, the sensor-based image stabilizer doesn’t work. The camera menu indicates that the lens does not support stabilisation. This even works with purely manual lenses after entering the focal length. As the EXIF data shows, the lens also transmits the correct focal length of 14mm to the camera. Well, with 14mm focal length I can probably well do without the stabilization, maybe SAMYANG will provide it in a firmware update.