When you say you are using the ATMEGA328PB, are you using the Arduino IDE and writing sketches with their library functions, or are you writing in embedded C using the Atmel/Microchip IDE?
If you are using the former, then you can move up to 32-bit ARM based products which are adapted to the same Arduino IDE and similar libraries, such as these:
If the latter, then you are limited to the Atmel/Microchip AVR product line unless you want to convert to something like ARM products or PIC32’s. Most AVR’s are limited to 20MHz instruction speeds, with the exception of AVR DA and DB subfamily family and the XMEGA subfamily, which can run up to 24MHz and 32MHz, respectively. However, the XMEGA’s are limited to 3.6V operating voltage.
While on that subject, most faster MCU’s are also limited to 3.6V or less, so you will want to keep that in mind.