Thank you very much for the answer.
It is a pity, that we did not met earlier for discussing the usage of the library.
I have no experience with the ArduinoBLE library, because it does not fit to my own environment based on software-timer scheduled state machines.
But I thought about using ArduinoBLE without my environment (about 6 month ago) and the main problem for me was to build a network with 50 devices transferring 10 measurements (6*16Bit) per second. I had no idea how to do this.
Now I nearly finished my own protocol with a master/slave structure (1 master, 50 slaves). The master is polling the slaves. With response time-out of about 1 ms I can control the channel quality and I am working on a channel switching which is fast enough to guarantee the measurement rate also in case of disturbances by other radio transmitters (WLAN).