How about an Enterprise Dashcam?
Consumer dashcam is high volume/low price.
Enterprise can be high volume/high price.
But enterprise models have different requirements. Perhaps this is too radical redesign and too much for you to handle.
Main unit should be a separate blackbox (think "Raspberry Pi" or "Banana Pi" for the size) that can be hidden from view.
I think this alone could make you a market leader. Instead of a small main unit that is camera and CPU and GPS and display and front camera and ... ... ..., which becomes overheated due to too much workload, you can make a dedicated main unit inside a blackbox that is hidden in the car, for example inside glovebox or underneath center console.
For enterprise use, this kind of form factor can prevent many kinds of manipulation (removing of TF card to destroy evidence, disconnect cable to hide illegal activities, etc.) by bad workers.
This form factor also gives you new possibilities while still remaining "Mini". The cameras can be connected to the blackbox via cable, so front and rear only has a small camera and no other distraction. This is very discrete.
Also the blackbox form factor allows you to provide a SATA port for SSD instead of using TF card. There would never be an issue with broken TF card again. SSD can support much more writing than TF card and has higher performance.
The display with buttons and GPS can be optional add-on modules that are connected via cable for the user who needs them. This design allows you to build a wide ecosystem of compatible add-ons (high quality cam, low quality cam, 360 cam, IP67 outside cam, big display, small display, battery pack, etc.)
With this form factor, you can support more than two cameras. For example, you could provide 5 inputs. (FRONT, REAR, INTERIOR, LEFT, RIGHT). Then it could be useful to monitor large trucks (LEFT and RIGHT is useful to record evidence of crash with bicycles who are not seen by truck drivers in the mirrors). Users who don't need so many inputs just leave it disconnected.
Configuration should be by WiFi and small web server. WiFi password can act as security. Please don't make an app. Apple and Google change their OS all the time, and you have to update your app or it will stop working in a few years. This is the biggest problem with Chinese products at the moment: the vendor stops to care and a product stops to work with recent OS. If you make a web based configuration with mobile design, you never have to worry about upgrading the app. All smartphones have a good web browser and will always have it.
Unfortunately there is probably no SOC that can support this kind of ideas (I have many more). You would have to invest substantial R&D to make such a hardware design.
But I'm sure there is a market for this kind of product, maybe even possibilities of OEM for large truck companies if you have the right connections.
Here is a simpler suggestion for the next model:
Consider a smaller camera connection cable. Many vehicles in Europe have a kind of rubber tube to connect the car body to trunk door. This tube houses the cables to the rear lights, etc. It is the perfect place to do wiring of a rear camera, but with the current 0906 connection cables, it is difficult to put the wire through this tube. Perhaps use Mini SMB connector to decrease the size of the connector as much as possible.