You could probably address 99% of the market only selling BTC, a stable coin, and small amounts of Eth needed for gas and then any other trading could happen on

You could probably address 99% of the market only selling BTC, a stable coin, and small amounts of Eth needed for gas and then any other trading could happen on dex's. And you could make that a modestly profitable business if you run a tight ship. But you won't build an empire doing that.