zdravke:
So I tried to write the efuse to 4 and it was successful. However, my bootloader is gone it seems, and I cannot find any bootloader that works for ATMEGA328PB. I tried several to no avail.
Are you not already using MiniCore, which has support for the 328PB? Out of curiousity, where did you get your 328PB Nano? I've been looking around for one for a while.