Do you have any snails in the sump? I've used the same float switch for 5 years and its never failed except for a snal that found its way into the sump (my fault)
The key is to use two float switches and a pump that wont pump so fast. A dosing pump on two switches is the way to go.
For now I'd put a needle valve on the end of the output line. Slow the feed rate down to a little faster then evaporation. The pump will run longer but if the switch fails it wont pump so fast.