What I do do is almost always seek out lux for lux trades, if that costs me 1-2 gpt on top then fine. Another way of converting gold to happy is through allying city states. But it's less efficient (1 lux can be sold for 240g, it costs 750-1000g to ally a CS for 1 lux)
Xiao Xiong is advocating passing on lux for lux trades and instead taking 6-8 gpt.