There are a lot of details to consider for large custom tanks.
If price is a minimal concern, most go with acrylic and are done with it.
When price is a top concern, plywood tanks seem to be a preferred approach.
While plenty of other products could work, there are reasons other options are more commonly used.
Though weight distribution is easily solved.
As previously discussed here, while the stump is certainly heavier than water (once the air voids are water filled). not by a shocking amount.
More so with heavy rocks than wood, point loads are a concern. But there are practical ways to alleviate that concern. Placing egg crate/light diffuser on the bottom is something many of us used to do.
If you're convinced that the total weight of the decor is more than the bottom panel can hold, consider a stand that supports the bottom panel. I've had several tanks that sat on a piece of syrofoam that was on a stand with the edges completely unsupported. I admit they were nothing in the 700 gallon range. But if all things are the same, only larger, I don't see why it would be any different.