This is an elaboration of a comment I made on olliebulle's answer. Shimano's carbon pads may not work optimally with other rims. That does not mean they won't work on other rims. Rather, rim manufacturers will specify the pads they think work best with their own rim (and I think it's mainly about the resin they use in the brake track). For example, Zipp has its own dry conditions cork pad, its own general purpose pad, and it also allows Swisstop and Bontrager pads. I have no direct experience with carbon rim brake rims myself, but this blog post by November Wheels asserts that "there is also a broad range of what works".
I have a set of Hed Jets (25mm width at the brake track) that works fine with R8000 rim brakes and normal width pads. This width may not work well with older rim brakes, as they weren't designed with this width in mind, and might not provide enough leverage - keep in mind that 15-20 years ago, 25mm external width might have been seen as an MTB size.
I italicized brake track in the paragraph above. For clinchers, if it's not stated but the internal width is, you can guesstimate by adding 4-5mm to the internal width. For example, my Jets are 21mm internal and 25mm external; I have a pair of Boyd Altamont rims that I think are 19mm internal and 24mm external. I note that you have tubulars, so this might be a bit different but it should still be in the ballpark.
There are very few rim brake wheels whose external width is at or wider than the R8000 calipers' specified maximum width. I recall that Light Bicycle, a China-based direct to consumer brand, had one rim that's 30mm external width - and I'm not sure who even would but that, and they don't list it now, so I assume it was essentially nobody.
