It's true that the 135 f2 + 1.4 extender (which I own) would have been a stop faster than my RF 70-200, but it would definitely not be 1/2 the weight. Just out of curiosity, I looked up the specs. The EF 135 f2 is 750g, and the extender is 225g, so the combo would be 975g. The RF 700-200 f/4 is an amazingly light 695g, so almost 300g lighter than the EF combination.

As mentioned above, I did end up going with the 70-200 f4 and feel that lens turned out to be the right choice. The ISO (in auto) varied from about 1500 to 4000 depending on the specific scene and lighting. The R5 handled those ISOs very well and any noise was easily dealt with in post. In retrospect, I'm glad I had the flexibility of the variable focal length because I was able to be much more flexible especially in the post event photos outside the venue.