sounds promising
drilling the holes was the hardest part... im partial to SPS tanks with 50-100x tank volume turn over so I would...
one of the 1.5 inch drains I would run at full siphon as a closed loop with a reefflow dart 3000-6000 gph returning via 2x .75inch returns over the top regulated by oceans motion 2 way... you'll need this to be 1-2 inches lower than the other drain, which you can do via an internal 90' elbow and strainer pointing down from the bulkhead
the other 1.5 drain I would cover with 3inch tall overflow box with teeth. It would gravity drain to sump/skimmer/filtration and return over the top via a slower reeflow pump (2000-4000 gph), split via 2x .75inch returns via oceans motions 2 way...
total of 4 returns...you would not need any internal powerheads.... ok maybe 1-2 vortecs if you're doing SPS...