I'm not taking it apart to change the design now, it was already expensive enough.
I am not sure where I'd put dowel rods either. They do sell much larger metal braces but they are also a lot thicker and would push the insulation out, I didn't really want that to happen so I went with these. I actually had even smaller ones at first.
I have used a few hundred 2.5" and 3" Deck Screws thus far, that is what is holding it together.
Attaching it to the garage isn't an option either, I rent.
I do appreciate the ideas, I just can't really change the design now that it's pretty much done and I've already spent like $400 or so...
The reason I think it will hold ok is
check out this pond.
Same design as mine more or less, and if that 8'x8' holds then my little 7'x4' should hold fine.
It is the same design, just bigger. So I think I'll be alright.