I have resealed many tanks, and dabbled in building a couple, if I can help you I will.
But honestly, if it’s that nice of a tank I’d seriously consider letting a professional take the tank apart and rebuild it. Taking a tank apart and basically building a new tank from scratch isn’t as easy as just resealing the inside of a leaking tank. Plus, I know I wouldn’t be happy with my expensive tank if the silicon wasn’t perfect.
If you decide to go the DIY route, my advice for separating the panels is to get a short piece of wire - a “high E” guitar string works great - and wrap each end of the wire around a short piece of dowel or broomsticks to make handles. Work the wire through the silicon between the panes with a sawing motion.
Once apart, you need to meticulously remove all the old silicon and clean the glass very well with alcohol or acetone before applying new silicon. Some well placed blue painter’s tape on each side of the joint will help you get nice, crisp silicon lines. You can smooth out the silicon with a little tool you can find by the caulk at the hardware store, you can also use your finger, but dip it in water first, makes it go a easier.
There’s lots of videos on how to hike tanks out there. Many could do this just with some masking tape holding the panes of glass together. If it was me, and given the size of your tank, I’d be building some type of wooden fixture(s) that would hold the panes of glass perfectly square.
For silicon, you can purchase aquarium safe stuff, but GE Silicon I (<Roman numeral 1) is totally aquarium safe, 100% silicon. Do not use GE Silicon II as it has mold inhibitors that are not aquarium safe.
That’s all for now off the top of my head, I hope that helps. If I think of more I’ll post again.
Good luck!!