Update: Carmax took the car for a price I couldn't refuse. Essentially only $4k less than I paid for it new 4yrs ago.
Conveniently, my Mazda dealer had a 2018, damn near same model, with 30k miles (Half of mine) and the Cerified Pre Owned warranty on it for $27k. So I'm out a few thousand but will be in a car 1yr newer with half the miles and a bit of extra warranty protection.
In regards to the 2nd mechanic. He was very sure there were issues inside the engine. He stood in front of my car as I made the sound and he instantly was concerned. He didn't quite say "It's done", but he did say he thought I might get 6months out of it before having more major issues. That coupled with metal in the oil convinced me that there definitely were some internal issues that were going to require a hefty bill. He wasn't even attempting to offer a fix, just giving his advice on the condition of the engine.
I think i would have gone further down the route of diagnosis if CarMax wasn't giving absolutely insane prices on used cars. I definitely didn't come out "Ahead" after all of this, however with the problem still being partially unkown, and the potential of an $8k fix and weeks in the shop, I decided it would be better for me to swap.
Mechanic did give me a good piece of advice, which was "A car is a hunk of metal, do NOT get attached to it".