A less-brutal Congo Free State, one that obtained ivory and rubber through trade for European manufactured goods and extracted revenues for Leopold through royal monopolies or import/export duties, still could have been profitable. It would almost certainly have produced far less revenue, but it would also have been much easier to enforce. If you're forcing the native population to produce rubber and ivory for you at gunpoint, you need to buy those guns and hire people to wield them for you. OTL, the
Congo Free State spent about 46% of its revenue (see Figure 2) on military and police, far more than other European colonies in Africa.
A minimalist Congo Free State that left the natives alone except for taxing trade with them would probably have been cheaper still than a standard European colony in Africa. The CFS corresponds pretty closely to the Congo River's drainage basin and access to the interior relied on the river, so trade with the interior could probably have been controlled by a small garrison in Boma (the capital of CFS, located near the mouth of the Congo) and some revenue cutters.
The profit to Leopold and Belgium would likely have been less than OTL's brutal regime, true, but there still would have been potential for substantial profit.