How to "shift" a logic LOW of 2,5 V to GND?
How about some simple bipolar inverters like below?







simulate this circuit &ndash; Schematic created using CircuitLab

With 5V at input, Q1 is turned off so R4 pulls the gate high, turning on M1 and your LED strip. With 2.5V at the input, Q1 is turned on which turns on Q2, which pulls the M1 gate low which shuts off M1 and your LED strip goes off.

You'll need to refine the circuit, check the resistor values, find a better M1, make sure response is fast enough for your PWM frequency.