Would a design using either PPTC or automotive circuit breakers be
OK I'm going to answer and say "No", PTC resettable fuses just won't cut the mustard. I'm basing this on a search of Farnell. I narrowed down the search by telling Farnell's search engine to look for PTC fuses that were rated at 60V or more. From memory, 60V is a decent enough margin in automotive applications to allow electronics to survive i.e. a 60V rating gave something reliability in the automotive environment.
Once narrowed down, the largest fuse available was 3.75 amps and this barely eats into the rangle of 3A to 30A specified in the question. Given also that these types of fuse seem to hit the end stops at 85C I would also consider them to be "fragile" in these applications.
Passive aging, thermal shock and humidity aging is something else that makes them look unsuitable but I'm no expert here. This specification gave no indication of how they cope with vibration either and, I'm sure this is important. Also, on page 1 of the data sheet (normally where all the intentional lies occur), "Automotive" was not listed or implied as an application.
My search is not exhaustive so I would encourage you to use my methodology to see if something looks like it might be suitable.