We had a discussion about fiberglass vs. carbon masts and support material in #hamradio antennas almost a year ago.
Since, I asked Martin DK7ZB, who has a reputation as an antenna person in Germany. He's done some experiments; iirc, he used horizontal carbon tubes supporting/carrying dipole wires. His results: Carbon stuff in parallel to radiating wire badly mistunes the antenna and produces losses. He recommends against that material.