Okay, I tested it enough and I can say that it's working as expected.
I removed the audio decoder box that came with my nifty city tesla screen, and replaced it with the harness I made. After replacing it and wiring the RCA to the tesla unit, the sound went back to it's normal volume before the install, also the hissing noise that I had is gone using the custom harness.
For the wiring, I used the pinout posted earlier in this thread.
I tin soldered the connections between wires and tapped it with heat shrink sleeve.
To remove the pins, there is a tiny hole in each pin, I mean, each pin has 2 holes, one for the connection, and another one tinier, which is for unlocking that pin out, so you can gently insert a small tool on that hole and pull the wire with the pin out.
I used this tool, which is for removing pins of PC power supply:
After that, and according to the pinout, I removed every wire that was not needed in Metra 70-7553 and 71-7552. Took the extra pins from the Metra 70-7552, because Metra 70-7553 lacked some pins that were needed for steering wheel controls.
Also removed the audio pins from the Metra 71-7552, because the audio is wired to the RCA from the tesla unit.
Here is the harness finished:
And here how it should be wired:
I finally wired the shield wire like a normal wire, no issues.
If you have spare audio decoder box and you dont mind breaking it, you can do everything with only the Metra 70-7553 and take the male connector and the rest of the pins from the decoder box.
Note that this worked for my car (2019 Nismo with factory NAV), but I checked the pinout on the AV manual, you should check yours before doing anything, I am not responsible for any problems someone else may have with this wiring, and this post is for informational purposes only.
Hope it helps.