Maybe like this?
Use the small powerhead as AQTCJAK suggested. That way the water isn't rushing through the refugium too fast.
Top view:
Probably not the right spot for the powerhead in the refugium.
And a side view:
This might be a better spot for the powerhead in the refugium.
So, you have an inlet and a return on the sump. But you just bypass water, at a slow flow rate, into the refugium and back into the sump. A bypass loop, I guess.
This way, you can have the flow that you need in the sump, but keep it at a lower rate in the refugium.