I just did the same thing! Love my 6D still but the AF is a shocking improvement. I was pretty good with focus on the 6D. It almost feels like cheating on the R. Can't imagine what the R6 and R5 are like. The 135 on an R would be heaven. THAT was a challenge on the 6D.
I had the 6D and got the R. When I saw some were getting great results with the 135L, and I knew I was getting the R6, I got a used one. WOW! The R6 with the 135 are unbelievable.