Found a BUG in my SPI Lib. So please don't use. I will upload a new one later. I work currently with this chip at work. I develop a Test- and Programming Station.
The boards with the 328PB act as slave controller which test and programs the target boards. So they need to talk to a Master and to the Target. Therefore I need two hardware UART. Software Uart is to slow.
In one test, the target (has a Atmega328P, running at 16 MHz) sends data with 115200 Baud. The function uses Arduino-like Serial.write(...). Weird is, that the 328PB seems to have a problem to receive at full speed. If I slow down by 400us between each write, it's working.

The Slavecontroller receives at Uart1, which is combined with MISO0 and MOSI0.

I will see more tomorrow.

Good Night.