I have been an electronics technician since 1978. NEVER use solid-core wire, or cables made up of solid-core copper wires, like Cat5, Cat5E, Cat6, etc where the wires will be flexing, bending, rolling around and just moving. Take an old metal coat-hanger wire or bailing wire or any solid-core wire, (not just copper)… bend it over and over at the same point and it breaks. ALWAYS use stranded wire, or cables made with stranded wire, for any application that requires the wire and/or cable to flex frequently. Solid-core wires, and cables made from solid core wires, are for things like AC power wiring or in applications where the wire/cable will not move once installed. Hard-break. End of discussion.
Also, the more strands and smaller diameter of each strand, the more flexible the “Wire”, or more accurately “Cable”, is.
For example, look at welding cables. Each of the two wires/cables are made up of small diameter, multi-stranded copper wire cables. I say cables because it is a bunch of small-diameter coper wires bundled together and twisted together like a rope and all of those tiny diameter wires touch each other.
That makes those cables last long, deliver a high amount of current to the job, and can bend over and over.