How much current (Amps or milliamps) do you need?
Your general approach(s) are good. I didn't totally analyze your circuits. A voltage regulator is the normal way to step-down voltage.
But, you'll need to get rid of the series resistors at the 12V input. The diode on the input is only good for reverse-voltage, and if that's going to work you need a fuse. Without a fuse the diode will get fried with reverse voltage.
Linear voltage regulators (like the ones in your schematic) are only good for low current less than 1A. And, it you approach 1A you'll need a heatsink. (The current ratings on the parts don't tell the whole story.) A switching (AKA switchmode) voltage regulator is more efficient and they can (generally) handle higher current. But, switching regulator circuits are not as easy to build so if you go that way it might be best to buy a pre-built module.
With the voltage regulators in series, the 1st one has to handle the current of both circuits, so it will get hotter, but the 2nd regulator doesn't have to "drop" as much voltage so it doesn't get as hot. So, the preferred method depends on how the current & power (power is related to heat) is distributed,