Nah I'm good with it. Big boy tank less than 24" tall is not the way I want to go. Whether I go 24" or 27" or 20", the 36" Width will be the challenging aspect of the maintenance.
I think you want me to get a shallow acrylic frag tank!
1/2" is plenty for this size tank but 5/8" would be a good option. 5/8" is about 1.5-2x the price of 1/2" and 3/4" is about 3-4x the price of 1/2". I personally don't think the 3/4" is worth the huge price jump but the 5/8" is.
Definitely do stainless steel 316 top and bottom braces. It's a bit pricier than 304LL but much better suited to maybe environments.
You can definitely do SS bracing with an external overflo. My 650g at the store has the external overflow integrated into the SS brace. Also, Wayne's tank has SS bracing with external overflows.