Mining is and will probably always be the most profitable way to earn credits, best not doing it alone though, normal trading, is more designed for supplying and RP purposes, though it can be gratifying to complete a far run
The maximum that a normal trade run can be made is about 20 million depending on cargo and distance
game doesnt need a rebalance at all, its balanced already
miners mine, pirates pirate those miners because of the smell of credits, traders fly and risk those pirates as well, systems are balanced for that