For one of the ESP32_Devkitc_v4 board, make erase_flash then make flash monitor unable to boot it up (show below). But if I briefly short GPIO12 to 3V3, /dev/cu.SLAB_USBtoUART would be disconnect and red LED dimmed, this time press En button would boot normally. If I short GPIO12 for too long. The /...

Once connected, the encryption is taken care by the BLE stack. The SDK example such as spp server can help to create your own GATT servers. I found web Bluetooth is the easiest to work with as the client. Good luck.

From esp32_datasheet, at normal speed it takes 25mA for core, 100mA for BLE Rx, and 130mA for BLE Tx at 0dBm. So the short answer might be no. EPS32 is not in the same 'battery optimised beacon and bracket' class of those BLE only chips... I'm more interested to find out the range when set BLE Tx at...