Naw, HDMI is just on the main board. But USB does travel through EXT board.
The CPi team is pretty helpful about providing these. I had an easy time getting the dxf files for the EXT board.
I encourage you to give PCB design some more time (if you have any interest)! You seem like you have enough knowledge to carry you. Don’t let high-frequency signals or gaps in your knowledge scare you away. Like you said, these are just tools.
I may not be more knowledgeable, but if you decide to start designing this, I’d be happy to provide some feedback if I can. I bet there’s others here who would also be happy to provide some feedback too.
I honestly hope we get strong enough community that eventually redesigns the whole Dev Term! The community already has lots of feedback and ideas for improvements.
If you haven’t already seen @SpicyRamenCo thread, they are working on plans to redesign the battery module: Reverse Engineering DevTerm Battery Module
They seem open to collaborating!