I think that Tom and Mike hit the main points that I have.
It isn't that bioballs and canisters aren't effective for filtration; it's that they aren't generally the solution that folks have long term success with in reef environments. In the nitrogen cycle (at a very high level) ammonia is broken down to nitrate by aerobic bacteria, and nitrate is broken down to gas by anaerobic bacteria. The high aeration that is typically caused by canisters and bioballs produces nitrate faster than the anaerobic bacteria in your system can break it down. The overwhelming of the nitrogen cycle with nitrate in the reef environment is why people refer to bioballs and canisters as nitrate factories.
As Mike said, daisy chaining multiple vessels is verrrry high risk. I would consider elevating the additional vessels or trying to use a larger sump tank with multiple chambers.
I would also recommend keeping the RDSB and refugium separate. They achieve a similar goal with different environments. Refugiums do well with a lower flow than a RDSB, and use photosynthesis in the macros to take nitrate and phosphate out of the water. RDSBs a sand bed full of anaerobic bacteria to remove nitrate from water that moves fast enough to prevent solids from settling and building up on the sand. The presence of algae usually traps detritus and overwhelms the sand bed. RDSB is usually dark with high turnover, while refugium is lit with lower turnover.