I’ve been working on this board layout and would like some critique and feedback. If it sucks, tell me how much and what I should be doing instead!  I don’t have a great deal of experience laying out boards and this is my first ‘proper’ complicated layout – everything I’ve done before has been fairly trivial and much simpler than this.
The line down the middle is where the board is intended to be split, so I end up with two boards that will be stacked (with component sides facing). By the way, ignore the grey traces between the 14-way headers in the middle, I had to put jumper links there to satisfy the ERC – they will actually be a ribbon cable when assembled. Might have it v-groove cut at time of manufacture, or maybe leave it whole and cut it myself – depends on cost (I know many PCB makers want to charge you extra for ‘multiple’ designs, which this isn’t really, but who knows what they’ll say). I would have made it a routed slot with break-away joins, but that would make the whole thing >100mm wide, which unfortunately puts you into a higher pricing bracket for a lot of PCB manufacturers.