Wow, with an 8" rise, that pretty much throws out the suggestion of going higher.
Extreme extensions put more pressure on the mount and frame.
A tongue extension would do the same. Take a look at the lowered recomended tongue weights with an extension. If it was just at the ramp you might slide by but 2' is going to change the distance you have to drive in by less than 2'.
The Sienna's low ground clearance is also not helping.
Maybe a $3k truck stored nearby to launch? We have several trailer boaters that leave an old truck attached to their boat an drive in with their car. They get to the lake faster, with better milage than towing.