Would be great for birding. If the AF is as good, or even better than the R5´s, then 7 fps would be great.
I expect 7fps, because data transfer technology moved on. And I heared one expert (other brand) talking about an bigger cache or built in RAM to store pictures ans send them to the card afterwards. Maybe Canon will do something similar.
The R5S will be - IMO - a lot more pricy than the R5 is. 5500-6500 Euros to expect. Just think of the fanboys with GAS? (Like me, if I can still hold an camera
)
BTW the pixel-shift mode is fine. I used it with the 5D4 with indoorshots of church domes and buildings. My oldest great-grandson won an photographic school-competition and his pixel-shift-shot was printed on an about 10m long tarp. and it looks great, as there you can see so many details.