Oh, an information I should have been posting before (sorry for that):
All Information comes with four equal predefined bytes, followed by four bytes which contain the length of the JSON object that follows those 8 bytes. (see here: Connect to a TCP Server with special packet order)
It is interesting, that the request TCP node nearly always splits the information immediately after the first 8 bytes and then - if the object is big enough - again after 1343 bytes.
Sometimes - but very seldom - there is only a second part that has up to around 14 something bytes, which is good to handle for me. Now I try to solve the third message parts...
@marcus-j-davies Thanks Marcus for this starting point. I'm gonna come back as soon as I stumble at the next point! 
