In regards to building it differently next time.... I would probably try a veneer finish, although I love paint.
Go order more than what you need of everything, maybe even a spare driver for an 'oops'. I really wish I had ordered another 18" TC LMS 5400 when they were readily available.
Research research, research as much as you can. I chose to go with QSC as my amp of choice, based upon many many hours of reading. Do it right the first time, overkill is OK, but you still need to occasionally move your creation.
Making a 3" thick box is way overkill. even for 4 18" PR
's, and two 18" active drivers. Mine is 1.5 with many large holes in it, and is still hella heavy.
With regards to bracing, the KISS principle is good here. A piece of lumber, ply or MDF say 3/4 x 1.5 glued on each large panel will do almost as good as a window pane style brace, and requires no extra effort, I have used scraps in the boxes I have built.
Double baffle is OK, but generally not required.