p.1 #2Is it worth upgrading from Sigma 150-600 mm C to RF 200-800
You will probably get the best answer the Canon camera forum. You will be trading the reach of an average performing lens to what might be a slightly sharper lens with a slower maximum aperture. I suggest you compare them at 600mm to understand the impact of the two lenses maximum aperture on the ability to separate your subject from the background.
p.1 #3Is it worth upgrading from Sigma 150-600 mm C to RF 200-800
I used the Sigma on a 5D MK IV and a 90D. I now use the 200-800 on a R5, R5 MK II and a R7. Way more in focus with the newer stuff. Images are sharp, but focusing is better with th R5 MK II (of course). Not a real big fan of the R7, but I have some keepers from it.