Sorry Im late to this party, but I just read thru this thread and still trying to process all the info. Well done Reburn, your thoroughness and taking time to think about your build will surely pay off. Since you asked me for some input, here are a few things you may want to consider:
Make sure the fabricator adds adjustable feet to the corner posts. No floor is level and feet are a good, cheap solution. Four of them will appear flimsy compared to the rest of your design but they can handle a ton of weight.
Consider removing the 5 floor braces and set your sump directly on the floor on a sheet of high density foam insulation. Structurally those braces do very little and you would pick up a few more inches of clearance and save a few bucks.
Along those lines, I would suggest moving any of the horizontal braces off the floor so the post are the only thing on the ground. Much easier to level and they won't end up sitting in saltwater when you have the inevitable spill.
Just curious, whats the purpose of the double braces at the bottom?
Long term Id be concerned about hot dipped galvanized metal holding up. Love the idea of the Rhino coating though.
Consider having the stand fabricator add some tabs to the back supports so you can easily bolt a piece of plywood to the back to mount odds and ends (BRS reactors, ATO controllers, Apex components, etc.) Avoids drilling holes in the frame exposing bare metal. Probably do something similar for the top braces so you can easily mount your fuge light (if needed).
Is the ATO reservoir a custom piece? If so consider slanting the bottom so if you add Kalk it will settle to one end for efficient mixing. I found flat bottom tanks build up old Kalk over time.
Now that I have a few frag swaps under my belt, I think the traveling frag tank is something you will find impractical. Moving your corals into a separate tank with different flow, lighting etc. for any period of time before the swap risks pissing them off and not looking their best. Plus you cant really transport the corals in a shallow frag tank with water without it sloshing out and knocking the corals around so youll be draining it before the swap anyways. I think youd just be adding more steps to get ready for a swap and you really want things to be simple and streamlined. Plus I like storing my frag tank dry and clean so it looks perfect for the swap. No coralline, salt creep or gunk to worry about.
Even with my architecture background, I find napkin sketches (no offense) difficult to really understand. I threw my best translation of your design into SketchUp to see what it looks like.
Here it is again but with my suggested modifications. I think the results speak for themselves.
General thought about this frag tank. Im assuming that the primary goal with this frag tank is to raise coral in perfect conditions, hence the long list of equipment which most of us have on our DT's. But isnt main for most of this stuff is to counter the heavy nutrient loads that fish create? It seems to me that adding fish to this system would be counter to the primary goal. If you go fishless, I'd bet you could get rid of bio pellets, macro algae (and associated light) and the GFO reactor. Maybe even get away with a smaller skimmer and less live rock. Less of a chance for a nutrient roller coaster ride which our precious SPS will not tolerate.
In any event I'm excited to see your build progress and feel free to ignore any and all of my comments without hurting any of my feelings. Also while your waiting for these things to be built, feel free to come over and I'll show you some of the trials and tribulations I learned with my frag tank build.