As implementation of my plan to continue to work on my measurement control solution, first I done the design of the USB-RS232 interface. It could be, that this is not the last version of it, but the design is done for now.
It is based on the CH340G chip what I used for my ESP8266 programmer with great success, also have it in various chinese USB-Serial cables, Arduinos, 3D printer boards and almost never had a single problem with it. To achieve the proper signal level I added an MAX232 to it.
Probably this solution is not optimal as it is lack of all of the RS232 signals, but I think it is good enough for now (I don't want to put much more effort into it for now).