I don't think you're getting my point, consider reading again. There's a fundamental limit you will hit here, you can't just "make it slower" or "make it worst" to lower that limit.
In the limit, If something takes 5,000 watts, you could run it for a few minutes/day, with that same RTG, provided you had suitable energy storage.
Perhaps they could gather grains of material, and just sort them, one at a time, only keeping the iron rich material, or use a permanent magnet to gather ferrous material. You could sinter the grains together using a microwave or laser pulse.
The results don't need great quality, just enough tensile and compressive strength to be mechanically stable during additive or subtractive manufacture.
Lots of minds have been thinking about refining metals for a very long time, but they haven't been thinking about doing it on Mars, with limited power, and very far outside the box of normal constraints, like cost.
This is one time capitalism doesn't apply at all... and most solutions assume capitalist incentives and costs, instead of going back to first principles thinking.