[.....]The higher number of free players will also flip the balance which might drop the value of the item that can be sold for Ethereum or other types of coins.
This is something that you should consider as well OP. I understand that the game you're trying to build does not require cash outflow to start but would still take time before players can create valuable items. Time is money as they say so maybe try looking at how you can balance the game. I mean would it still be worth spending an hour playing the game for a few cents? I get it that there will be those who would play it completely for fun but let's face the fact that financial incentives is what drives a lot of gamers.