Ultrasonic transducer driver/receiver ultralow power issues
The linked paper from TI (slaa136a) has been updated here which also includes firmware and schematics file, it could be use as a base.

To achieve low power you could disable the boost convertor which drives the TX stage when your firmware puts the MCU in sleep, Q1 will isolate the MCU power rail from the TX power rail when in sleep. Then since not powered CD4049UB will not draw any power.

On the receive side LMP7715 will draw 1.5mA from the 3.3V rail. As you indicate powering this from  GPIO could work, you'd need to make sure that you have enough decoupling for dynamic response. MCU noise should be out of band for what you are trying to measure.