the esp32 with poe is pretty cheap

Which make & model?

Throw it all in a plastic workbox.

Not sure where u/Logical-Meta is located, what their standard workbox/switchbox dimensions are.

POE looks nice but might require a bigger box as ESP32 with Ethernet looks bigger? Also, I’m not sure it will be able to power the mmWave sensor? I only saw 3.3V output. Any recommendations?

We are building mmWave sensors using the Olimex ESP32-POE. This board has both 5v and 3.3v available on the side set of pins.

Yes, the PoE boards are a bit larger than USB-powered devkits, and the price is a little higher – felt the extra reliability justified spending a little more for a device with integrated PoE versus adding complexity with WiFi or separate power supplies for each sensor.