Yes, soft is the right phrase. It finishes very well, but boy is it ever soft.
But strong is not how I would discribe it. I have used it for a number of items, including a foot stool. There I had to go with stock that was about 2x as big as pine.
The wood is used (in Japan) mostly for ceremonial use, not practical use.
As for it being harvestable in 5 years, I doubt it. Thirty is more like it. Yes it does grow very fast, but that fast has some issues. One is the the wood has a hollow core all but the most mature wood has. I have a 3 year old truck (from one of mine that died) that the central opening is about 1/2 inch across.
Another is very large growth rings, which is part of why it is so soft and light.
In my climate (west of Seattle), a tree grow about 5 ft per year, for the first 3 or 4 years, then slows to less. At 10 years it's down to about 1 ft per year. I know in warmer/wetter climates it will grow more, so much more that several states in the SE have declared it a noxious plant.
I don't have 2 acres worth, just 5 or 6 tree's, but I do know the tree well (BTW, if you live in my area and want a root start, I give them away)
TTFN
Ralph