Everything Dies When I Go to Start SOLVED!

Hello all,
I finally have the Crusty Biscuit at the body shop for paint! Everything was working fine up until I went to start the car to drive it up on the trailer.
Here’s my issue:
I turn the key on without starting and everything works; the fuel pump starts priming, Holley sniper display turns on, and the compressors for my air suspension system turn on. BUT, when I turn the key to start the engine everything dies! I can’t get anything to turn on immediately after that but if I wait a couple minutes it will all come back on like normal.

I haven’t done much to troubleshoot the problem but here’s what I have done:

Charged battery overnight, it reads 12.2 volts on my sniper efi display

I tried jumping the starter relay but that won’t get the starter to work

Checked and tightened all power wires and grounds

I feel like the issue is starter related, I recently removed the starter wires to install a heat shield on them to protect them from the exhaust. It worked fine several times after that but of corse when it’s show time something decides to goof up!
Initially I think the guys above suggesting bad battery cables or clamps are correct, but you need to define "everything dies"
When you hit the key...does all power go away, and it's as if the battery isn't even in the car? No horn....no sniper power...no lights.. nothing

Or...

Is it cranking, but just won't start?