This is essentially what I have been doing since going from the EF 24-105 f/4 L Mk 1 to the EF 24-70 II. Never looked back. Even on the 5DIV. The IQ out of the EF 24-105 was good. But I've always preferred the IQ (contrast, sharpness, etc) from the EF 24-70 II. I do understand that the RF 24-105 is better at 105, which may change the RF conversation a bit. But, as Joel points out, you are still getting a 17-20 MP image even when cropping on the R5.
Great point. I have reassigned my DOF preview button to toggle between 1.6 crop mode and FF (a Joel Eade suggestion, ).