Fascinating stuff! I am also an electronics engineer so if you need any information or help, let me know. I have some contacts in manufacturing and the boards could get made in the UK if you desire, for a BOM cost of about $60 (taking a guess considering connectors are pricey) manufacturing would be around $120 to $150 total for 100-off boards at the company I work with which deals mostly with low quantity runs. (Of course pricing is in GBP there, converted for convenience.)
If I were to suggest minor changes, you could move to putting a TVS on the 12V DC input. I know that ICE cars are incredibly noisy and have transient issues due to the alternator and ignition systems. This is of course less of an issue for the Tesla but it's probably a good idea anyway.
I'd also suggest a fuse in addition to the polyswitch because those polyswitches have been known to burn when subject to long term fault overcurrent. You could use an automotive blade fuse in a blade fuse holder.
Also, if you're doing production runs, it's a good idea to get a controlled impedance board made. At high quantities the cost isn't significant, compared to the additional cost in handling returns from customers whose boards do not work due to marginal board tolerances. Typical tolerances on those cheap boards can be ±0.1mm. It looks like you used Laen-PCB to make those, judging from the purple colour. They're a lot more consistent than the cheaper boards made in China, but they still have significant tolerance errors.
How much current does the board pull in operation?