Whoa whoa whoa. What's going on with this new tool? I would practice it on something else before trying it on the big boy.
I would echo your concerns about the structural silicone. Is it coming out in little frays/chunks from any of the seams when you are cleaning? Or look cracked or otherwise funky? I haven't seen a Pic of the bottom seam from the side--can it be seen with that trim on the tank?
One of my first concerns when I was looking at your tank is that there was another issue besides seals that was causing the leak. At this point, if it's leaking from that same area every time, I'd say there is a weak area in the structural seams that keeps springing the leak every time.
Tbh this would be the largest glass tank I've ever seen resealed by about 2x. We talk about this procedure pretty casually but in reality I'm not even sure if a DIY reseal is even a thing to do on a tank this large. It's almost a requirement to tackle a project like this in a professional workshop, sterile environment with no shortage of skilled helping hands. I'm getting more and more nervous every time you fill that thing up, at some point an 800g tank failure can go very scary.
Of course I'm cheering for you though and I'm sure you'll get it this time. What's that they say about third times? I think 3 weeks minimum to cure but you'll have to closely inspect that super fat seal for squishiness anywhere before proceeding.