Sorry, I have no answer for your question to the 512 Bytes. But ...
I am interested to hear/read more about your real-time transmission.
I am developing my own library for a real-time communication on Nano BLE 33 (bypassing mbedOS and ArduinoBLE) based on non-connectable and non-scannable advertise-pdus. My aim is to transfer 6x16Bit (acceleration and gyro) with 10 measurements per second for each of about 50 devices (distributed multi-channel measurement).