Repotted two of these in March. Only one has forgiven me.

March isn't the problem, drainage usually is. If the pot has no hole at the bottom nothing else matters 😄

True.

how do you actually tell it's overwatering and not something else, is there a sign before the leaves go

Stem gets soft at the base first. Check before the leaves.

soft as in mushy or just bendy? mine feels bendy near the soil

When did you water last?