Eureka, finally found a way to prepare it – cold brew. It is absolutely lovely (to my taste buds) that way. Minty, but without bitterness and some rooibos body. With a bit of brown sugar it was even better.
Normal amount of rooibos, cold water, put it in the fridge for a couple hours, added a tiny bit of brown sugar. Might be the most stomach friendly drink ever – rooibos AND mint.