I think there are two things to consider(possibly 3) when shooting action and getting the moment from my experience so here me out
1. Buffer capacity(this is how much the camera's RAM/CPU can capture shots in its internal memory before saying(nope cant take anymore)
2. Storage speed(This is related to how fast you can get the images from the ram/cpu into the storage, uhs-11,cf etc etc)
3. Frame rate(self explanatory but if point 1 can only hold 25 shots yet your camera can shoot 25fps, your buffer is filled in a second and doesnt matter if your writing to a ssd mk4 type interface of a floppy disk)
Point being, for your issue in the confetti shot scenario, The 5d4 is lack on all 3 fronts and that the UHS-1 Vs UHS-2 is a smaller issue for you in the grand scheme of things.
This is why i reckon the A9 would suit your needs well because it does 1 and 3 exceptional making the issue with one slot being recorded at only uhs-1 not a massive deal as you will find it difficult to fill the buffer capacity(241 shots i believe) and you will nail the shot fine at 20fps, or even 10fps(giving u more buffer time).
The D850 has issues too despite nailing my 2nd point.
what is that? The buffer capacity. It is only 51 shots when shooting at its maximum detail(
https://photographylife.com/everything-you-need-to-know-about-the-nikon-d850)
So my point is that have dual CF or whatever is not the be all end all. You need the write mix of the 3 points i have highlighted