I don’t think any of the methods are going to stay secret, not for long anyway, like you say.
Ancient Essence. It takes a long time to craft, but if you already have a lot stockpiled, it is worth a decent amount if you are not going to be using it for anything.
Large Fossils are easy to come by, and a mass craft only needs 18 fossils (0.3c/u * 18 = 5.4c worth) to make 250 ancient essence (2.8c/u * 250 = 700c worth) The caveat is requiring power and a very long crafting time.
Medium Fossils, alternatively, require 18 fossils (0.5c/u * 18 = 9c worth) to make 100 ancient essence (2.8c * 100 = 280c worth). They require no power, only spark, and less time, but you can easily have more machines processing medium fossils if power is a difficulty for you and spark isn’t.
Not the best, but it’s very passive and guarantees an income if you can be bothered to set it up. *Well, until any mint value changes occur of course. 