Yeah, the Tesla subwoofer even with premium sound is only OK, so I can understand the motivation to better the subwoofer. For what its worth, the premium audio is already really well tuned for the car, Tesla did a good job of EQing the baseline.
You'll need the Tesla rear amp though. The signals that go to that amp are digital, not analog, and are encoded somehow. So no one knows how to use them to get audio. You thus need to tap the outputs to get analog to input to something like the JBL or Alpine DSP.
Still- with the premium audio system, this will be a bit of a mess. You'll need the front channel audio signals that you can only get from the computer in the front of the car, and the rear channel audio from the Tesla rear amp. You need all those to do a proper job of getting the input to the DSP, otherwise you'll only be able to tune half the audio, which sort of defeats the purpose and is too much work for low improvement.
I think if I were in your shoes, I'd try using the stock rear amp, and just wire up your better subwoofer and see what you think. Simplest and fastest way to improve the premium system. (you can get the subwoofer analog audio out pretty easily) Leave one of the rear seats down to really improve the bass. At some point before 2020 Tesla removed the ported area in the rear deck and it degraded the audio.
My car is the lowest cost $35K unicorn from 2020. Base audio, and I don't even have AutoPilot. So I only had to tap the front amp in the computer and could map the 6 channel audio to my JBL DSP inputs. I used the NVX subwoofer and custom box in the driver side rear cubby. I use a 600W JL Audio to drive the woofer in the doors, and the subwoofer. The audio is killer, once tuned with the DSP. Only remaining problem I need to solve is that my 920W of new amps will throw battery errors about once every 3 months.