Off camera flash, R5 viewfinder always dark

[gui]

For a few months, I have had a Canon R5. I use it for videography and portrait photography. I pair the R5 with a set of Profoto flashes. I place the camera on a tripod to make a portrait. I always used a DSLR.
Because the DSLR has an optical viewfinder, you could always see what the scene would look like through the camera without modelling lights or permanent lights.

But the R5 uses a screen. For documentary and reportage-style shooting, it Is a blessing. But shooting with flashes is a disaster, in my opinion.
Based on the camera settings, I have set the viewfinder to show me the scene. This means that when using natural light, the screen shows you the correct image in the camera.
But when I set the flashes and configure the camera to reflect my proposed image settings (often low ISO, Large aperture and 1/125th because of the flashes), the viewfinder is permanently dark. I can set the viewfinder to show me the screen all lightened up, but I only want that for my shots where I use flash.

How do you solve this?

Is there a setting that I can program into a button on the camera that can toggle between the two settings? Or temporarily turn it off during this particular shoot?

 

[neuroanatomist]

How are you triggering your flashes? On my R3, I have the viewfinder display set to exposure simulation with DoF preview. When I am doing indoor portraits, it’s usually with a backdrop and I am at exposure settings along the lines of 1/160 s, f/8-11, ISO 200-400 (with the intentionally dim ambient light, the settings mean a black viewfinder).

I use a combination several 600EX flashes and an optically-triggered monolight. I trigger with either an on camera 600EX or the Canon ST-E10. If either of those is in the hotshoe and powered on, the VF automatically brightens for full scene visibility (but only when metering is active, i.e. after a half shutter button press). If nothing is in the hotshoe or either of those attachments is mounted but powered off, the VF is black consistent with the exposure settings.

 

[gui]

I trigger my flashes with a Profoto connect.

I have the same setting—exposure simulation with DoF preview. And I have almost the same settings for my images. But my VF does not automatically brighten for full-scene visibility. Is that something you can set in the flash menu?
I want a button to configure to toggle to full VF brightness and Exposure simulation.

 

[neuroanatomist]

I trigger my flashes with a Profoto connect.

I have the same setting—exposure simulation with DoF preview. And I have almost the same settings for my images. But my VF does not automatically brighten for full-scene visibility. Is that something you can set in the flash menu?
I want a button to configure to toggle to full VF brightness and Exposure simulation.

AFAIK, there is no setting to control that. Possibly the difference is that I am using OEM products in the hotshoe and you are using a third-party trigger.

I don’t have a third-party flash to test that hypothesis, do you have a Canon flash to try?