The barrel design of the Thinkwares are not the most stealthy of designs, so yes people will probably notice it more than the most stealthy ones. ( this do depend some on car make/model and windscreen layout )
For a cabin camera you will want something really small, but the problem is when the light get low cabin cameras start to struggle really bad.
In the daytime everything are just fine and only worry will be how large FOV lens to get in relation to placement of it.
But night time you would do well with a camera that are able to go into B&W mode and be aided by IR light, but you could also just rely on the short period where the cabin light are on, this should still give a okay capture as people get in.
If you can increase the time the cabin light are on, that might be a good idea, maybe bump up the strength of the cabin light bulb / bulbs.
There are memory card extenders, this normally go from the micro SD cards dashcams use but then up to a regular sized sd card in the other end.
But i think this are mostly used if you bury the camera so deep you cant easy get to the memory card in its default location in the camera.
The little Mobius 1 also have a extension to the lens / sensor part so you can remove it some inched from the main body ( size of a match box )
This might be a good option of you hide camera body in A pillar
This will make what is visible minimal, and you might even be able to change lens to a wide angle pinhole one and then put lens module in a faux tweeter or something on the dashboard.
The sensors in the little old Mobius 1 cameras are not the most stunning low light performers, and it is optimized as far as it can go.
BUT ! There are no real parking guard in these little cameras, they do have motion detect as i recall, but its not a pre buffered one, so by the time it get going recording the cabin light in your car might have dimmed / shut off.