Honestly I wonder if the REAL issue is the temperature shock from washing it with cold water right after brewing. I used to do that ALL the time and went through three presses in a year before I figured it out. Let it cool down completely before you even THINK about rinsing it, the glass is way more fragile when it's hot.

You ever try cranking the tension before the sagging became a problem? I keep mine just tight enough to hold at rest but loose enough to move by hand, never had a sag issue doing it that way.

Flip the stone around and try sharpening with the other side up next time.