If you are planning on shooting greater than 6K on the URSA12K I would suggest considering the Zeiss zooms.
My Canon 100-400mm, which is a pretty decent zoom wildlife lens will not show significant difference from 6K to 8K to 12K. My Zeiss Otus 28mm will clearly show difference of 8K and 12K.
You can also go Fujinon, but for that level they cost more than the Zeiss.
I also had an opportunity to try the Tokina 25-75mm cine which is great at 6K with some improvement to 8K and then no difference to 12K. Sigma zooms might work similarly (I have not tested), but I would suggest the Tokina over the Sigma just for the lack of breathing.