I am not aware of anyone trying this method with an octopus (but have seen success with corals with fish) but I suspect there are reasons why that you have not yet considered (especially around minimizing the water change thinking). The water quality for an octopus is, unlike corals, not related to the balance of trace elements but very much an issue with regard to waste and waste by product removal. Think a very large, messy fish or eel. Their main diet will be crustacean (typically table shrimp, crab claws, small crabs, mussels and occasional fish), all things that pollute quickly (recall that shrimp are often used to create enough ammonia to cycle a tank without fish) and don't filter easily because of the massive amounts of ammonia (hence the need for a very well cycled tank and preference for live rock over or in addition to other biological filters).
Deeper sand beds too can be an issue for harboring pollutants so a thinner (with a few exceptions because of in situ habitat) rather than deeper and keeping it well stirred is better suited for the type of water quality needed for our messy wards (
my currently active tank for ET is bare bottom with a small DSB in the overflow and only filtration in the sump. My other two, unoccupied octo tanks still have a small amount of sand as bottom substrate but I am slowly removing it after finding this tank easier to keep balanced without).
Cuttlefish are an alternate ceph you may also want to consider given your experience and tank design. You still have to worry about heavy ammonia with cuttlefish but they are more tolerant of corals, you can keep several in your tank, no octo-proofing is needed and they are a favorite of many. The initial couple of months after hatching (obtaining them as eggs tends to be the best way to find them) is expensive as their only commonly available first food is live mysid shrimp. The shrimp don't live long, have proven impossible to raise in enough quantity and are quite costly to have shipped in every week so take that into consideration but look through some of the cuttlefish journals to see if they interest you.
Octopus and cuttlefish both have very short lifespans. Octos will 99.99% of the time come from the wild and will be 2-10 months old. Cuttles will be eggs collected in the wild (there are programs to produce them in captivity but they are still fledgling operations). If you design the tank for an octopus, you can later house a couple of bandensis cuttlefish (the most commonly found animals in the US, none are native to the Western hemisphere) and switch back and forth over the lifespans of your animals.