After reading some of the pros and cons of screw in anchors, you might look
at your local utility company poles. Around here (SW Wisconsin) they use
screw in anchors to guy utility poles when going around gradual curves, at
corners where they change directions, etc. I dont know how deep they put
them or how big the screw plate is but I do know that they tension some of
these tight enough to BEND a pole sticking up 30 feet or so above ground.
Over the years these poles still have the bend in them and the wire they are
holding is still tight. So they will work but might not be for everyone.
Piece of mind is a BIG factor. 73 Paul K9OT