Ben,
I did not realize you had decided to order an R5? Very cool! Who did you order from - do you have a delivery estimate yet?
I had been very happy with my EF 16-35 F/4, until a very rough moment when the contents of my camera bag ended up strewn across the concrete floor at the DIA security checkpoint about 4 years ago. My 5DsR landed on the corner of its L bracket, and came out unscathed. My 70-300 F/4-5.6 L IS was visibly broken. My 24-105 F/4 L IS was on the camera and seemed to be untouched as well. But my 16-34 F/4 L IS, while appearing fine physically, was never quite right after that. I initially sent the 70-300 in for repair and it came back good as new. Eventually, after noticing softness in the corners that seemed worse than when I got the lens originally, I sent in the 16-35 too. It came back, but the problem has largely persisted. I have still taken some good images with it, but I am hopeful the 15-35 will outperform it in many ways. The extra stop is of less importance to me, and in fact if an RF F/4 version that was lighter and smaller were available yet I would opt for that instead, although I think at F/2.8 the current RF 15-35 could be pressed into service as a nightscape lens as well. It appears to be extremely good on star shapes in the corners wide open at 15mm...
ML