For many years, I have used the 18" Shoot-N-C targets as a practice tool out to 600 yd. I tape a target to the cardboard insert that comes in the target package as a support, then black out all the lines and numbers with a fat black Sharpie, leaving only the red aiming dot in the middle. I also use a compass to inscribe the "9", "10", and "X" rings in the target face, which are also readily visible through the rifle or spotting scope. I can see bullet holes at 600 yd using this approach, both .223 and .308, out to 600 yd. Past that distance, you really want a camera system or a ShotMarker. Even with the increased contrast/visibility from the Shoot-N-C target face, reliably spotting bullet impacts past 600 yd is difficult, especially if there's much mirage. I buy sheets of the round black stickies to cover up the bullet holes after each shooting session. I can usually get at least 4-5 uses out of a single target setup before it's shot out.
The only real issue with the Shoot-N-C targets at 600 yd is that you want to limit the number of shots per target face to around 10-12. Otherwise, it can become difficult to conclusively identify the latest impact as the number of bullet holes increases, especially if you're shooting out the 10-/X-rings as F-Class shooters tend to do at 600 yd. It can also be a little difficult to see the impact when you hit the aiming dot, but it's usually not impossible to figure out where that shot went. I attached a few images below as an example of what they look like, on the first of the dual targets, there are both .223 (L) and .308 (R) holes. The other two targets are .223 bullet holes only.
I ordered a ShotMarker back in March and am hoping it will be here soon. FWIW - you don't necessarily have to make a gigantic frame for the ShotMarker. Because I will currently only be using it out to 600 yd, mine will likely be 36" on a side (haven't built it yet). Having used the 18" Shoot-N-Cs for years, I believe a 36" target frame at 600 yd will provide plenty safe margin for the sensors, yet not be so bulky or troublesome to transport as would be a much larger target frame. Obviously, I'd want it to be a little bigger at 1000 yd LOL. A shooting buddy uses a target cam system, which also works very well. Either one will allow shooting at much greater distances than the Shoot-N-C targets.