Why signal deteriorate in half-duplex UART communication in long wire (6 meter)?
Use a higher transmission voltage (I've seen 12V used successfully with a very similar half-duplex bus in a commercial product with an operating range of several hundred metres) and smaller pullup resistors (2K2-4K7).  Also, don't expect high data rates; I'd suggest 4.8kbps as a maximum (the commercial product uses 4kbps).