Of course. I bought the R and turned it over to my daughter. Although it was rather short she shot with the R and my 70-200 X1.4 in Costa Rica and had great results when not FLL. I'm not the least bit worried that the 400 DO II with X1.4 won't be just fine with the adapter and if I then have 45 MPs I think that'll be quite acceptable. There is a good chance that with the bare 400 I will have decent reach, especially for bears.So I guess you are going to bring a trusty old EF lens to go with that R5, since there aren't any RF lenses suitable for bear country.
As far as balance goes, I shot for years with the 300 2.8 II with X2 on the 6D and that wasn't all that bad, not ideal. I'm not thrilled about the weight of the 1DX2 but like the size - always trade-offs.
However, I doubt the world will be very well by this fall, somehow.