Is a 'timeout' of 0.2 ms for UART communication realistic?
This totals less than 200 us together... Do I miss something?


In case the other party is a PC: USB timing. This is usually scheduled in full USB frames (for full speed), and can add up to about 1.5ms of latency. Even more latency is possible when the PC is "loaded".

In case the other party is another µC, your 200µs seem to be a usable timeout.