This is a great discussion. It took me a while to get used to the R's EVF when flash was attached, because it defaulted to Exp. Sim. Disabled, which, compared to OVF, seemed disorienting because everything was always bright in dim rooms. (So looking through EVF kind of threw off my visual cues for settings and bounce direction.)
I've always been disappointed with the R in this regard. I've just tried all the relevant options. The instructions are the same as for the R5 (or very close). See page 248 of the EOS R Advanced User Guide. But please note/remember that the R does not have a dedicated DoF Preview button. (In my case, all the DOF Preview does is make things more grainy and jerky, plus show DoF.)
No matter which option I try with the R, when a Canon Speedlite is attached and powered on, the EVF seems to compensate for what I'd call "visual comfort." In other words, as I move it from dark to dim to brighter areas in a poorly lit room, the EVF adjusts to let me see clearly, but it has no connection to my camera settings whatsoever. No change of camera exposure settings does anything to the EVF brightness. The EVF becomes a simple tool to let me see and compose, but offers no help with ambient exposure while flash is attached.
I always thought this is some kind of bug, and I've learned to just get my visual cues for flash settings without depending on the viewfinder. In fact I've come to trust the R in these situations, as things work out and it does seem to have great ETTL going with Speedlites. But it is a jarring adjustment coming from a 5DIV.
So I'm reading in this thread that the behavior of the EVF with flash attached is still not as expected.