Our spa is under our deck. We had very little choice in the location. It's adjacent to a 10' X 10' patio, which I did not want to place the spa upon. No other nearby space was acceptable, for one reason or another.
The bottom of our deck joists are about 9' from the ground, and I don't really feel closed in. We have enough of a sky view from the sides, and we have a casino which blasts light pollution in our west sky, so we aren't missing much anyway.
We don't have too much of a problem with water dripping between the deck boards. We considered one of those under deck drainage systems, but the cost was too much for me to stomach.
Occasionally, a light dirt line forms on the cover from rain water dripping thru the boards in a certain spot, but regular cover cleaning takes care of that.
OTOH, we don't get near your rainfall.
Overall, it's fine. Not perfect, but plenty good enough.
Brewman