Buck converter output
Using a buck converter as a pre-regulator to drop the input voltage for a linear regulator is a valid approach and way simpler than a full regulated buck converter. An unregulated buck converter avoids a lot of the headaches you'd have with a regulated one.

You can avoid the output voltage rise without load by replacing D4 with another N-MOSFET. This FET should be on when Q3 is off and vice-versa. Make sure to include some dead-time so you don't have shoot-through between the FETs.

With those two FETs (that form a half-bridge), energy can flow back from the output capacitor to the input of the buck converter when the voltage at the output is too high, allowing the converter to always operate in CCM mode and giving you the desired output voltage even with no load. As a bonus, it will also be more efficient because the voltage drop across D4 will be avoided.

As Bimpelrekkie already stated in the comments above, you should probably drop the voltage to about 8V for the 7805 and use a second identical buck converter to create a higher-voltage rail.