I have little doubt the R5 is going to be an excellent camera. While the specs are great, I think it will be great as I already own the 5DIV and it is a great camera and I expect the R5 to be better.
The R5 solves my two primary issues with the 5DIV but also introduces a few other issues. The two issues that will be solved by the R5 are fps and AF point spread. And, I do want to dive past the 12 fps, as I often fill up my buffer when shooting with the 5DIV. Right now I am assuming that I can eliminate this issue by writing exclusively to the rumored CFE card. So, it is not just the 7 fps vs 12 fps, it is the size of burst I have that fps for. AF point spread, IMO, this is the primary benefit of mirrorless. But there is a benefit of using the 80-90% of frame while looking through the viewfinder.
The negatives, I have tried on a couple of occasions, EVFs are ok, they work, and they even have some benefits but I have not used them to the point where I like them. At this point, I still consider it a negative. Then, I am not one of the photographers clamoring for more MP and really think 30 MP is a very nice spot. I know all the arguments, memory is cheap, etc. The counter is I do not like working with large files. They slow everything down. So, I am actually hoping that the R5 is 40 MP (which corresponds to 8K horizontally) and not 45 MP. Final negative is right now I can't use all those RF lenses. So there is no need for me to buy them. As soon as I have an R body, I'll start being tempted by the RF glass but I love my EF lineup. I know I can use adaptors, but I've tried using the M system as a beta for mirrorless and have learned I do not like adapters. It is another step in swapping lenses, it is more caps to keep track of, etc. When I go R, I will drop money on the RF lenses that I most often use.
Then, directly comparing the R5 to 1DX III. First, we still do not know much about the R5. I am excited to hear about hands on experiences. But, I expect Canon to sufficiently differentiate their 1D/R1 line from the 5D/R5 line. AF speed? Battery life? Build quality?
So, add that all up, and I may be making the opposite decision. I am still waiting, but if I had to pick today, I'd order a 1DX III and solve my high FPS and buffer issue. BTW, I am a hobbyist, so this is not an assessment from a professional perspective.