I've made some very large vivs. Prior to my current plexi/glass/wood setup
I had large plywood + glass and both those vivs followed a few previous mid-size setups with plexi and plywood.
I have not worked with Pond Shield. I used West System epoxy as well as a Sherwin Williams potable water epoxy for my builds. I notice pond shield instructions mentions applying it with "a squeegee or roller" which makes me wonder if the consistency is rather thick. West System epoxy has a very paintable consistency which is very, very helpful when coating a large, 3D plywood surface.
In terms of dimensions, I would suggest more ambition in the vertical. If you are going to the trouble of building such a sturdy stand, I would push the viv's height to 36". You can still get adequate lights for that height (vivs any taller are more challenging for lighting).
Aquariums tend to stay wide/long rather than tall because reaching an arm into a deep tank is a hassle for setup and upkeep. A front-opening viv does not have that limitation and you will appreciate the extra options a taller viv presents you in terms of plants and decor.