All I can tell you is I've encountered borers from time to time and it's a toss-up as to whether the tree will survive. You've got some big ones! Reminds me of the borers we sometimes get with hornbeams. Those trees either die outright or stay weak forever. Bald cypress is also sometimes attacked by a borer (similar size to yours) that tunnels beneath the bark but doesn't make a "shot-hole" like on your redwood. We detect those by a crimson sap oozing from discrete spots from under the bark. Once we peel the dead bark away we have interesting tunnels. These specimens often survive, though with dieback, and can actually make interesting bonsai with deadwood. The low holes on yours are concerning. Maybe give the systemic a try. Don't see where it can hurt.
