Dear @Harry_Dogg,
in the linked thread, I've posted some code of mine (app + ino+ an explanation) on how to make a double connection, BLE and Classic BT, between an ESP32 (2 boards) and an Android device (a Lenovo pad featuring Android 9).
Please give it a look, probably you can find some help. For example the "register for string" is made only once after the app initialization, by means of a clock, which is disabled after its first event. Since you want to use the BLE only, you don't have to care about the .ino for the ESP that is running the Classic BT, but anyway, I hope the rest can help. Best wishes.