I agree it is definitely a stretch for a full mold, but was thinking along the lines of an mold pocket insert, like those which can be 3D printed [1].

You could make this from reasonably sized stock with 2 setups. First you rough the shape and cut the mold cavity, then you remove the stock and face the backside. I'm used to an automatic tool changer, which would be needed if you want to go fast

https://www.pressebox.com/pressrelease/apium-additive-techno...

Beyond the cooling jackets there isn’t really going to be anything hard to mill. If the geometry is hard to mill, then it’s almost certainly inappropriate for injection moulding (assuming you actually want to extract your injection moulded object from the mould).