Thank you! Also maybe useful is adding an LTE modem, a USB UART serial adapter and other usb devices (bluetooth audio in my case): Adding LTE Modem and other USB devices
Today I added another small thing to my reform: a second 3.5 mm audio jack:
Adding the hole to my 3D printed side panel for the 3.5 mm TRS jack and soldering left and right audio to the existing TRRS jack was easy. What turned out being tricky was to solder to the ground of the existing TRRS jack which was covered by some yellow goo. Even after scraping that off, I didn’t manage to solder a better connection than one with 7 MΩ… So in the end I connected the ground to the GND pin of the Line In connector next to the TRRS jack which worked much better. 
Advantages of having one headphone jack on each side:
- I can choose which one to plug my headphone in depending on where there is more space on our narrow couch or in the train
- I can now plug two headphones in at the same time which allows watching movies with another person without having to find the headphone splitter first and without waking the baby up