I feel we need a reality check here man. Not trying to be mean, or disagreeable. This is just my opinion based on owning a 3s Granite, which is basically the same as a 3s brushless Typhon, which is far stronger than your Typhon Mega. And our Granite doesn't even handle 3s well. I would never try running it on 4s unless I was just doing speed runs.
You're trying to take a car that wasn't even designed to handle 3s power, and run 4s on it. TunedFrog has already tried this after upgrading his and has told you how fragile it still is, even with the upgrades he has done. Regardless of what motor and ESC the original owner put in it, it's not going to handle 4s unless you are just doing speed runs. Meaning no on and off throttle, no bashing around, and high gearing, which will put far less strain on the gearing and other drivetrain components. It'll likely only hold up in straight line runs, easing on and off the throttle.
You are already finding how expensive this is going to get, because you have basically smoked the entire drivetrain. By the time you get all your parts upgraded, you can probably buy a 3s Typhon and just swap your motor/ESC over to it. Although like I said, unless you're building a speed runner, this 6s motor would be detrimental to the longevity of the drivetrain, even in the beefier built 3s model.
The first things I would suggest along with a completely new spur assembly is at least find some metal diffs. Stock diff gearing for the 3s Typhon is 37t. RCAWD seems to have a lot of upgrades for the 3s Typhon, so I'd maybe start looking there. A metal driveshaft, metal CVD's, and all new bearing set is some other stuff I would get.
With all that power, I am betting the original owner didn't even adjust the slipper from the looks of it. It was likely slipping with the onroad tires you have on there getting a lot more grip. That generated a ton of heat and melted it. I wouldn't be surprised if that heat didn't transfer to your diff housing and soften the bearing journals.
One other thing I would like to add is for you to go on Amain and look at 6s RC's. Look at how much aluminum is used in the chassis and other components. And most of them have 7075 aircraft aluminum chassis. They weigh a ton compared to 3s models, and are built so much stronger. Now look at your Typhon Mega which is 90% plastic. It's not designed for that kind of power. The plastic flexes, causing gear meshes to go out of spec, creating wear, heat, etc. And no matter how much you want to run it on 4s, you're just going to keep throwing money at it to keep it running, albeit for brief moments before it smokes the next part.