Unless someone here is an ophthalmologist, or optical scientist, I suspect you will get personal experiences. I would suggest contacting an optical professional body to see if they can provide any such publications. In the meantime, by doing a short search, I came up with this academic paper:
Retinal Viewfinder: Preliminary Study of Retinal Projection-Based Electric Viewfinder for Camera Dev...
You will likely have to pay the fee to get the study, but if you are concerned about your eyesight, it's a good investment. Given the degree that these technologies are applied, if there was a significant issue, there would be several studies specifically focused on reported risks or drawbacks. Sometimes no news is good news...
So, with that effort aside, your question is, to me quite a broad one. My first response would be do you have reason to believe that your eyesight is vulnerable in some way to using an electronic projection screen from a camera? Given that if you use a smartphone, tablet, laptop, monitor or LCD TV, you are looking at one, albeit with varying distances from the screen, but likely for extended periods. There are concerns that viewing smartphones and tablets for extended periods is having impacts on the physiology of younger people who have done so most of their adult lives. There is an academic article on this one:(PDF) Effects of Electronic Devices on Vision in Students Age Group 18-25 (researchgate.net). It should be noted that this is focused on a generation that has grown up staring at these devices for extended periods, so they have an adapted physiology to this. It is far less likely to be an issue for older people whose physiology is established and unlikely to be impacted to anything like that same degree.
So, are you concerned about the technology in general, or do you have a specific risk factor to consider? Is your concern that you may experience motion sickness due to moving the camera quickly while looking through the EVF? I note that while you have said you are considering getting the R7 you have not indicated the type of photography you engage in and that can have a significant impact on how much you depend on a viewfinder. For example, shooting landscape, where the camera is likely to be static vs. shooting wildlife or sports where one is often tracking moving targets. The more specific you can be the better response you will get.
That said, people experience different levels of motion sickness whether looking through an EVF, OVF, VR display or out of a car window, so likely your reaction will be specific to you. Since that is a phenomenon you would experience quite quickly, I would suggest visiting your local camera store, or a photographic society to ask to check out the experience using a modern EVF for yourself.
From my own experience - FWIW: I an 70, have got an astigmatism in both eyes, they have been operated on and had optical insertions for cataracts, and been lasered over the years. Having worked heavily in IT, I spent most of my adult life looking at computer screens and displays of one kind or another. As regards photography, I have been shooting through a viewfinder (optical since about 1980, and electronic since about 2013). I always use the viewfinder, but I don't stare through it for many minutes at at time - in my own experience, which is mostly shooting wildlife and scenic, it's not practical for stills photography. I think you would find that many stills photographers look up and over the camera frequently to get a wider view, then through the viewfinder when they have isolated a target. This close and far switching is arguably more restful to the eyes than staring for hours at a computer screen or cell phone at the same distance - it is certainly in line with best Health and Safety practices for using display screens in business - something with which I had some experience when working in IT.
I have shot with the relatively primitive EVF units of Canon PowerShots, the M5, and now the much more sophisticated R5, 6 and 6MkII, so I think I can say I have had a lot of time looking through one. These latter allow a screen refresh rate at a higher value, giving a much closer approximation of what one would experience with an OVF. At no time have I experienced any eyestrain or discomfort from doing so.
Given that videographers tend to use the LCD at the back of the camera and use the camera at arms' length and/or with a gimbal, and likely a monitor screen, in which case you are using the LCD, I would suggest that the risk there is likely no worse than viewing a tablet or screen and watching a movie. Again, movies tend to be shot in clips and then people view other things while they set up the next shot.
So, I would suggest that to a degree at least, much more will be learnt from experiencing the use of the R7 for yourself rather than asking for feedback from others - for each of us there are significant variables in personal physiology, subject type, technique and camera models.
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
