Can I repurpose an unused 6" radon pipe as an electrical conduit?
It depends.  The pipe is not listed as a wire way or conduit so it would not be allowed for individual conductors you know that by your question.
If you limited the number of cables using MC for example it could be allowed.
However more than 6 current carrying conductors may be flagged since it is basically just a hole not a listed gutter.
Would I do it as a professional? Yes even with NMB if I cut the pipe so it is completely enclosed in the wall.
Code allows wires run in walls with the finish or covering and doesn't require stapling.
This is even safer for NM, or MC wire types than just in the wall but since it is not a wireway we have to limit the number of conductors and use cables that can be run in the walls, then limit the number so the inspector won’t call out bundling.
Fire block may be required to plug the openings since this bypasses all blocking.