Electromagnetic waves have perfect/lossless superposition, so radiation can’t really degrade a signal that way.

The big limiting factors are free space path loss and noise.

That's only true in classical electrodynamics, as it happens. If you're in a very strong B-field like you might find near a compact object you'll get nonlinear QED effects.

You can get a low order correction with Euler-Heisenberg: https://en.wikipedia.org/wiki/Euler%E2%80%93Heisenberg_Lagra...

It should be same logic we use for repeaters, so it'll be fine.

The logic we typically use for repeaters (EDFA, erbium-doped fiber amplifiers) for long-distance lines amplifies but does not clean noise (so across the oceans, you are very much bound by SNR). And you need one of them every 80 km or so in typical fiber.