My familiarity with float switches is limited to boats. Any you would recommend for this application?
I just noticed that you're using a mains-powered pump, so don't splice its wires directly. Instead, insert the float switch on one of the wires between the QT Py and the relay.
Something like this: https://www.supplyhouse.com/Beckett-Pumps-1502UR-Float-Safet...
or
https://www.supplyhouse.com/Little-Giant-599124-ACS-3-Auxili...
I just noticed that you're using a mains-powered pump, so don't splice its wires directly. Instead, insert the float switch on one of the wires between the QT Py and the relay.