's (partial water changes) are all you need to keep nitrate levels in check. You want to keep them below 50ppm though fish can survive higher rates.
I don't see the reason for a biowheel (produces nitrates) after the bioball chanber. You would be better off using submerged LR
rubble instead of bioballs to keep nitrates low.
without much surface ares won't give you any real nitrate reduction.
A refugium coming after everything else is a good idea!
However, you are overly complicating the entire process.
You should do some more reading on the subject.
Everything You Always Wanted to Know About Sumps, Part I
For some good designs check out Melev's Reef - Sumps & Fuge's
And . . .