How to power an Atmega328P and Pi from 12V?
It looks from the comments you stated that 1.5A at 5v is not enough for you so It would be better to look for some of the higher power usually isolated DC/DC converters for example : 15w dc-dc converter will give you (3A at 5v) and are readily available though not cheap around 15-30$ meanwell SKM15A-05 .

However using a linear regulator is next to impossible , since the heat loss is around (12-5)*2 = 14W!! for 2A load , and (12-5)*3=21W!! for 3A load . and thats alot of power , you will need a heatsink the size of the respberry Pi and a fan to cool that .

There also the option to use 2 stage , 1 DC/DC converter 6v for example followed by a 5v linear regulator if you need to have clean power rail with less noise ..

have fun :)