the microwave sensors i see are several times more expensive than PIR, they're used for commercial occupancy sensors since they can be calibrated to keep lights on even if there's no movement, but a person in the room.

i haven't seen any of the cheap kits that include a microwave sensor + wireless receiver, because the sensors themselves are ~$100 and that's out of the market range for the little kits.

https://www.amazon.com/Leviton-OSC20-M0W-Occupancy-Multi-Technology-Self-Adjusting/dp/B002NU6M5M/