Are there poles available that won't slip under a heavy load?
Yes. Take a look at the Black Diamond Flick lock or the various other "me too" designs from Leki (SpeedLock) Exped (?) and Easton (RockLock).
If you are a heavy person look for the longer type (53-55") . At 5'7" I use the longer type simply because they are more versatile for my tents
as well as extra length for the downhill bits without having to extend them fully.
Franco