Nice tank! That sums it up via pics imo
Do as much vol water change as you are willing to do, matching temp and salinity. Suck out most of the sand that you can get to and rinse it hard outside the tank, back to white and then reinstall it (thin sandbeds are easy to work with) the sand will look brand new and have no waste, no clouding, and still retain its bacteria after the rinse.
the corals you have are forgiving of every known brand of water used.
Regardless of your overall export modes chosen, it's worth it currently not owning Api test kits until they're refined. Any data they generate is wrong and only a benchmark (a correct reading now doesn't mean a correct reading in June of next year for example) we can see from search results, it's not anti Api stance it's merely an aggregation of many api threads to steer tank concerns into overdoing export, water changes, preferable to any single parameter tweaking for the ends you want.
if less water changes and parameter detailing is your mode we should invest in salifert, simple as that