Maybe you should just order the Marumi kumquat from UCR/CCPP budwood.
I went to their field in Riverside last weekend with the CRFG and had a tour of their field with many of the varieties which were all fruiting. They gave us the tour and discuss the varieties, then let you pick the fruits to eat there. I tasted their Marumi and it was sweeter than the Meiwa, I was surprised. I did not taste much sourness in the flesh. I have the Nagami, Nordhmann, and Meiwa but never added the Marumi since the fruits are small. But now, I think I will need to buy some of budwood from them. Fruits are smaller than the Meiwa but seems to be sweeter, at least sweeter than my fruits.