I am going to +1 what wknd said. Park a truck on it it is fine. Maybe even a little overkill. If all else fails. Make it 3 to 4 inches larger than the metal stand, so it will distribute the weight better. But I think it is good. And in your last pic, I believe your stand is the wrong way around.
But that is my OCD.
I got most of my info on the drylock from a few reptile sites of people that use the foam board in their humidity loving reptile and amphibian enclosures. So, not sure what else you would have to do. If it were me, I would just use the grout and some silicone over it. And make sure you seal all the foam edges to the glass really well. It would not be good to have something from that leaching into your tank.
Hope you get better wkndracer.
And I am loving the foam background on this build so far.
Keep up the good work man.