I also have a Sigma EF 150-600S. I love some of the images I have captured it. The way it renders an image is great. But fine detail? It's ok/good, I used it for years, but I do not think it is keeping up with modern lenses.
Here is a post I did that included it while testing out the RF 200-800. Maybe not the best for the Sigma, but I would rank most of the other lenses above it. I move off of it as my primary bird lens as other lenses did better with fine detail. Of course, you may have a better copy.
As for a 300-600 f/5.6, I'll judge it for sure once I know more, but my first impression is that is a fairly limited focal length range for somewhat minor aperture gain. Compared to two existing lenses the RF 100-500L is only 2/3 stop difference, the RF 200-800 is 1 stop different at 600 mm. This is where it gets a bit funny, a 200-600 f/5.6 L is a lens I would definitely consider. A high quality 100-600 f/5.6 L I'd start throwing out superlatives, likely selling some gear and transferring money. But 300-600? Yeah, there will be a market, but I instantly think back to a trip to Alaska where I went from photographing bears at 600 (with the Sigma) to whales at ~200-300 mm. That flexibility is important (to me).