16A Relay controlled with PWM
Feed a square wave from your MCU to this circuit: -



It naturally blocks DC due to the 1 uF capacitor at the input but converts the AC waveform back to a DC level. So if you feed 3.3 volts p-p you will get a DC level on the output of 3.3 volts minus two diode drops i.e. a DC level of about 1.9 volts. If you use schottky diodes it might be more like 2.2 volts.

Next, use a MOSFET with a gate source parallel resistor that would naturally discharge the 10 uF capacitor should the MCU stop sending a square wave. You need to pick a value that doesn't discharge too much between MCU square wave cycles but does discharge in reasonably quick time when the square wave is terminated or lost.

Use the MOSFET to activate the relay but choose a MOSFET that has a low gate-source threshold voltage circa 1 volt.