I didn't want to get involved... Hey man, then I'd have to read pages and have to THINK & understand, and probably review that every time - and I don't even review threads I'm active in (hence my stupid relies 'cos I'm confusing my threads)!
Besides it seems like things are getting there...
Just re power from "the cigarette lighter connector (behind the dash)", that sounds like a connection to the socket's wiring - ie, NOT using a cig plug.
I was merely going to caution that (1) cig plugs can often be knocked or vibrate so power is lost & (2), my usual caution about some not handling more than a few Amps (despite alleged 15A ratings) and thereby a loss of entire vehicle (thru fire...).
PhilG makes a good point about ACC going off during cranking. Now if a 2nd battery is used...
You could keep power on during cranking - eg, a diode from STart, but probably to a relay used by ACC for your power straight from the battery for cleanest 12V and independence of other circuit loadings, or because you might need an RC delay to keep ACC on while moving to crank/STart (the ACC line should be disconnected BEFORE the starter motor gets its signal - ie, ACC & STart should NOT overlap).
Or you could power on AFTER the engine has cranked - eg, the UIBI as used for dual batteries - but you want ACC power as well. (Ok, a UIBI into a 'latching' relay (SPST with feedback) which can also be manually controlled from ACC power etc.)
The above depends on when it is to be powered eg, anytime, or IG key only; but not during cranking, etc.
As I recall the M-series specs have VERY good performance and should ride thru cranking - ie, they operate down to 6V. (I often say automotive stuff should handle (if not operate) from 8-16V but down to 8V can be very difficult. But that M2 6V spec exceeds even my own design criterion!)
Any switched-mode regulator/converter should clean up noisy or changing input voltage variation hence a direct fro battery feed may not be important. Most are switching at a few hundred kHz or even MHz (I think that mini converter I posted is 1.2MHz.)
And yes, the M4 can be problematic, but I also read of some M2 noise issues.
And I'd prefer one regulator supplying the end equipment (reliability, efficiency etc), but one as a pre-reg for others can also make sense - eg, you have the perfect splitter but it is 12V as opposed to automotive 12V. (LOL - I can confirm that "12V" LED tapes are 12V. My 5050 SMD license plate LEDs fried - sometime the series resistor; sometimes the 5050 LED itself. And yes, I I suspected that outcome.)
Anyhow, must shower and work....