HI There
I don't think Pi is needed at all in this scenario, Pi takes ages to boot, Pi needs a proper shutdown or else SD card is likely to corrupt
I recommend alternative to Pi, that is ESP32, I am sure you have heard of it
ESP32 is dual core, powerful and yet the cost effective IoT board with both BLE and WiFi SoC on board
ESP32 can connect be configured to connect to a network, ping to your web API to see for RFID authentication and engage the door/gate accordingly
ESP32 has lot of GPIOs pins to connect to door's/gate's relay/actuator
I have intense skills in programming ESP32, send me a message for more discussion please