I suppose I need to answer the question about trading my 6d away for the 7d2 or keep it for back up.
But if the 5d4 really is as good or as fast focusing like the 7d2 I suppose there is no need for it next to the 5d4.
Even with the improved pixel count, I could crop 1.6x to make up for the wildlife advantage.
Try the 5D4 in situations where you would use a 7D2. In my own experience, the 5D4 was more than capable. I did have a 7D2 for several years before upgrading to the 5D4 and 1DX2 last year. I sold the 7D2 to help fund the others, and I haven't wanted it back. That isn't true for everyone; it's just my opinion.