So theoretically, there is presently no way for mirrorless EVF to improve on bounce flash photography in the near future?
Do not confuse "improving flash photography" with "improving the preview image".
Theoretically, there is a lot of room for improvement.
The camera can use a very low power measuring-flash, and see how it affects the image. If the measuring-flash is low enough power, it may be barely noticeable.
The camera can use AI to track the subjects in the frame, and lighten the preview for various parts of the image in order to show what they will look like if a the flash were fired for real.
When the camera loses confidence it can fire off another low power measuring-flash.
Now this requires a lot more computational power that you typically find in a stand alone camera.
The flash must pre-fire and bounce off the ceiling to give any preview result of the image in the EVF?
That's certainly a good way for the camera to learn about how the environment will reflect the flash.
So presently, when you look into your Canon R5's EVF under low light social function indoors, you see a slightly grainy low light image, with not as much magnification as a Sony A7R V, and you just have to do a test shot with the bounce flash to see what the image looks like, and adjust on that shot if necessary?
With any current model camera, if you want to see what the flash is going to do, you need to actually take a shot and review it.
Similarly, if you want o preview depth of field, or motion blur, your best bet is to take a shot and review it.
As to low light preview, that depends on the camera and the lens. An f/1.8 lens is going to give you a a less noisy preview than an f/5.6 lens.
However, the true beauty of the mirrorless EVF comes up when you shoot without flash, and in such cases, the EVF will give you a preview of the final shot as you adjust your aperture, exposure and ISO?
Yes and no. First of all, the preview is limited by the dynamic range and resolution of the viewfinder. The preview typically happens with the lens wide open, so it doesn't show depth of field. The live video makes it difficult to judge motion blur.
However, it does give you a good idea of how dark or light the camera produced JPEG will look.
If you are doing any sort of custom processing of the image, the EVF probably won't match your results and likely may be a detriment. Imagine shooting a backlit model at the beach. With an optical viewfinder, you can see both the model's smile, and what's happening in the background. However, the scene may have more dynamic range than the EVF can handle. Either the model's face will be lost in shadow, or the background will be blown out. If you are manually adjusting the tone curve when you process the raw data (or using fill flash), the EVF doesn't reflect how it will look.
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Now I am not saying that EVF is bad. Only that it has both advantages and disadvantages. If your concern is previewing the lightness of the camera produced JPEG without taking a test image, then they are very helpful. Do keep in mind that the human visual system is very bad at judging how dark or light something is. Your impression will be dramatically swayed by context. An EVF preview that looks bright in a dimly lit room, might look too dark when viewed outside. If you really want to judge image lightness of the camera produced JPEG, it's hard to beat the histogram display.
