Welcome to the forum and to Roos. Congratulations on doing your research first!!
We are a family of five, too, and own a 233S so maybe I can give you a little insight.
I tow with a Toyota Sequoia as spec'd in my signature below. It has similar ratings and dimensions although I do not know enough about the QX56 than what is listed on paper. There are a bunch of ratings and weights for you to consider and I will point to just one that might be a problem for you: The payload of your QX56 needs to be watched carefully and you will want to run the numbers yourself using the actual weight of your SUV, passengers, mods and cargo.
Here's a towing calculato
r to help.
Personally, we are within our ratings with our Sequoia and the 233S, but barely with the payload now that the children are older. You mentioned wheelbase of 121". We are at 122". We are fine with that wheelbase. But, I recommend you replace your OEM P-rated tires with LTs. They help quite a bit to provide more stability.
Don't be too concerned about leaks with a hybrid or any trailer. Plenty of us have never had a bunk leak. Some have and they tend to be vocal about it, which is their right, but its not the norm. Good maintenance is the watchword with leaks.
Regarding the 23SS. I recommended the 233S for a family of 5. The additional bunk is really nice. With only two bunks in the 23SS, you will have to pack them all into one, have one sleep with you or setup the table or couch each night. That becomes old really
fast (did that for years with a popup). Plus, its nice for us to be able to sit at the table at night while the kids are sleeping.
The 233S is 50lbs heavier and the slide-out is different, but the beds would be more important to me having three kids. Think it over.