Ideally you want a board with Ethernet wired and wireless. Wired Ethernet for connection to your network and wireless for Bluetooth without interference from WiFi. See the advice here: https://esphome.io/components/bluetooth_proxy/#improving-reception-performance
Being able to power the board from a PoE switch is also an advantage.
The Olimex ESP32-POE-ISO-EA is a well regarded board for this and I have found it also to be faultless.
There’s also some great advice in this topic Great ESP32 Board for an ESPHome Bluetooth Proxy Read the whole thread, not just the first post. Posts by BDraco and Clyde Stubbs in particular. Both very active in ESPHome development.