Steady is fine - we usually run steady whenever possible. Your issue then is stability. The problem there tends to come from DPM source terms, so the averaging and linearisation terms can help. Depending on the coupling effect increasing the DPM update to 20-30 or decreasing to 3-5 can help: it depends what the DPM sources do to the calculation.
As mentioned earlier if the mass exchange is very low you may find there is an imbalance as you'll be running out of significant figures in the solver.
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