My .02. In reefing I have found to KISS Im other words keep it stupid simple! The more we throw into things the more “tinkering” or messing with it later always comes into play. I had that exact dimension tank previously and (4) 1.5” drains are completely overkill. Depending on the pump your running and what your overall goal is for turn over, 3 is plenty.
The likelihood you would ever need a clean out is next to never. I think there are several tried/tried methods for plumbing our reef tanks that would suit your build perfectly.
If ur into just trying something “different” then go for it!!! Half of the fun in reefing is the design and creativity that goes along with it. Just plan ahead, lots of unions and leave yourself an out of way to fix if it does not work like you plan!
Whatever route you go. Have fun!