It's not just the light level of the screen. Lighting in the room makes a big difference. If, say, you're in a dark room staring at a bright screen, it's a bit of a constant assault on your eyes. Also, it's
very important to get up and wander away from the screen occasionally - specifically, get your eyes looking at something that is farther away than a couple feet. Go outside (or at least look outside). Your eyes have muscles that control the focus from up-close to infinity. If you're staring at a screen for long periods without a break, your racking the focus nearly all the way to one end of the scale and tightly holding it there. Giving yourself a change of view isn't some psychological trick, it has a positive physical effect on your eyes.
Don't look for technological devices to let you keep doing exactly what you're doing in exactly the same setting - instead, change what you're doing a bit (get up and wander around occasionally, staring off into the distance), and change your setting (google for information on ergonomics and proper room lighting for screen use). That screen cover you link to advertises itself as blocking UV light (great, but there isn't
any coming from your screen), and blue light ("Night Shift", or f.lux for older systems, does this by cutting down the amount of blue light generated in the first place).
Another thing to try, which is similar to using Night Shift, is to go into System Preferences : Displays : Color, and calibrate your display. Change the "Target White Point" from (its likely default of) D65 (a
color temperature of 6500 kelvin, equivalent to noontime sunlight, and by its nature very blue) to a lower point, say D50 (5000 kelvin). You can save this as an additional "display profile" so you can switch back and forth as desired.
And, as craig1410 mentioned, consider getting an eye test. It may turn out that glasses with a very mild correction (or even off-the-shelf reading glasses) might significantly reduce eye strain while you're looking at the screen. Lubricating eye drops might help as well (especially if the air in your environment is fairly dry).