Canon RF 100-500mm f/4.5-7.1 as an upgrade to 70-200mm f/2.8 for sports?

The RF100-500 is a pretty fantastic lens in terms of image sharpness with a 5x zoom range, and focus speed. It also will focus very close for a telephoto lens (3 feet or 0.9m focusing distance with 0.33x magnification). So it is a good outdoor sports lens where the distances can vary drastically such as with baseball (Pitcher, batter in the box, batter running, and the fielders).

My (and many others) only big complaint with the RF100-500 is the way it works with the RF TCs. For example, if you ever get say the 1.4x TC you can't go below 300mm x 1.4 = 420mm. So you are limited to a 700/420 = 1.66x zoom range). On the good side, having the 100-500 is like having an extra 1.2x range over the typical 100-400mm. For people that just want the long focal length, it is OK, but for people that need the dynamic zoom range, it is a big problem.

Thanks to the ISO performance of the R6 (I would highly recommend looking at using RAW/CRAW with DxO with Deepprime for noise reduction), you might get away with it for indoor sports but a 70-200f2.8 would be a better choice indoors. I think the RF100-500 would be better than 70-200 plus a 1.4x TC in overall performance (sharpness and focusing while giving up only 1/3rd to 2/3rd over most of the range).

I don't know about Sigma 150-600 (contemporary ~$900 or sport ~$2,000) other than there have been reports of focusing issues. Some say the new firmware for the R5/R6 and or Sigma lenses might fix it but I have not seen this verified (see: https://www.dpreview.com/forums/post/66153095). I think in any event, the RF100-500 is likely sharper and will focus faster, and is lighter (about 25% lighter than the cheaper/lighter contemporary model) which will help with handling.