If you've loaded up the board via ICSP, it doesn't have a bootloader anymore.
Since you've gotten that far, I'd suggest installing the MiniCore core which has support for the 328PB, and burning the 328PB bootloader (with UART0 selected). Then you can use the board normally through the IDE. Almost. You won't select Arduino Nano as the board anymore, but MiniCore, ATmega328, 328PB variant.
Just a note for the future: I'd advise against trying to use Wire1 though, until you verify that SDA1 and SCL1 aren't simply tied to ground and 5V respectively (as they would be with a PCB designed for a 328P). I imagine it's unlikely you'd be able to use Wire1 in any case (unless the PCB has the pins brought out somewhere), but enabling those 2 pins as outputs with them tied to power... ouch.