I will turn my comments to an answer, though it is still a comment.
"I'm trying to estimate ultimate maximum space travel speed, assuming a spacecraft must have radiation shielding made of matter, no unobtainium force fields or suchlike."
In this link one can see that at relativistic energies the ions penetrate deep leaving a track of destruction in the material.
The dynamics of track development due to the passage of relativistic heavy ions through solids is a long-standing issue relevant to nuclear materials, age dating of minerals, space exploration, and nanoscale fabrication of novel devices.
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Track size and internal structure depend on energy density deposition, irradiation temperature, and material composition.
Random ions in the interstellar space at rest with the cosmic microwave background, if the velocity of the spacecraft becomes relativistic , would get radiation tracks through the shielding, in addition with any sputter at the surface. This could damage electronics, people etc. depending on the density of ions the spaceship would meet. Off hand I would say that the velocity of the space ship should be kept below relativistic to avoid this extra danger.