What is the best way of manipulating strings and using them as void*?

Mbed OS
3

you can try the std::string_view. It works well with BufferedSerial or UnBufferedSerial.
std::string message= message = string1+ to_string(float1) + string2 + to_string(float2) + “\n”;
std::string_view buff(message);
blutooth.write(buff.begin(), buff.size());