Timber extensions for a brick fence

I am extending the height of a 1.5 metre existing front brick fence by 350 mm. I have already extended the height of the brick piers. My next step is to span timber horizontally 3 metres between piers. This timber will probably be painted the same as the brick fence (grey). Merbau I presume would be best but painting it will defeat the looks of the timber. Treated pine is the cheapest but I don’t like the reeding on the timber surface I am also concerned with warping across this 3 metre distance. I could bolt a short timber post to the brick wall at 1.5 metres between these piers. From what I remember cypress pine can be bought non-reeded. Any suggestions on what timber to use?