Accidentally hit the redline on the on-ramp with my 21 GLE 53 because I didn't realize I was still in manual shift. I've redlined many previous cars, most just bounce and burble at the redline. This hesitated and then pulled all the power like a full fuel cut until I downshifted. Is this how the RPM limited in this car behaves? It's different than what I am used to and it honestly made me question if I popped it.

Yeah, that's what they do. Once you hit redline, the fuel shuts completely off for a full second or more. I expected it to just bounce off of redline, maintain power and just stop accelerating, so the fuel cutoff was a surprise.

At the track, I can't use manual because of that. The first time, I felt like my head was going to launch through the windshield because I expected an upshift.
The manual upshifts are painfully slow, and I can't find a shift point that accomplishes an upshift before the "rev limiter" stalls the engine. Even when early shifting at 4500 rpm, it doesn't shift before 6500, where the cutoff happens.

The plus side is that in Automatic mode, the shifts are crisp and at redline, so I (embarrassingly) track the car in Auto.
Downshifts are appropriate, shifting where the next-gear-lower is right at redline, with rev matching. It makes me look and sound like I know what I'm doing. I program the "I" button for Pro or Race in all parameters, though.

I just don't use manual mode. I haven't found a situation where manual is better than full auto. Even with those nice paddles.

It is what it is.

It's a surprisingly competent track car, though. The AMG Ride Control makes this 5,200 lb beast feel much smaller.

Yes, that's normal for this car. It's how the ECU protects the engine from over-revving. Unlike a traditional fuel-cut limiter that bounces, many modern AMG/Mercedes models use a hard cut that feels like a complete loss of power until you change down. It's jarring but intentional. Your car is fine.