I don't think I'd use bioballs. I'd either make that first compartment big enough for a skimmer, or make some sort of contraption for filter socks.
I think I would go with the skimmer, and the center area for the return, with the refugium (though after months of reading, concluded that refugiums are often times too small to really make a difference).
My sump: Skimmer --> Heaters --> Return. That middle section, I could easily use for timeouts, or toss some rock in there, for some pods with some snails/hermits, but for now it's just a heater housing section.