Which RF comms solution allows unidirectional short distance low data rate, low latency communication to the largest amount of receivers?
BLE or Zigbee are overkill for this.

You're making this far too complex, you do not need to individually connect to each receiver that's why BLE will not work, it is not intended for such short connections.

You would be far better off letting all receivers receive all data but in the data include an address indicating for which receiver the message is intended. All other receivers would simply discard the message.

Maybe you can make this work with simple 433 MHz (or 868 MHz) transceivers. These are cheap and simple. These use OOK modulation, you will have to do the bitrate calculations yourself to see if this will work for your application. The "intelligence" needs to be in a microcontroller in each receiver as this microC will detect the address. There is an Arduino library for such wireless RF setups, have a look at that to see what you can do with this.

And also: you do not need two-way communication, you only need one-way. Another reason not to use BLE as that is designed for two-way. That would complicate both transmitters and receivers as they all need to be transceivers (to be capable of two-way communication).