which SERIALx_BAUD port are you using, confirm it with the Mission planner parameter setting. double check your serial/Uart wiring, RX (Pi) to TX (Ardupilot), TX (Pi) to RX (Ardupilot). Dronekit seems to have stopped maintenance since 2020. give Mavsdk-python a try to see if it is the issue.
How do you verify your connection is healthy? try a print after connection to see.
try this and make sure you know which RPi dev port number you are using.
from dronekit import connect
# Connect to the Vehicle (in this case a serial endpoint)
vehicle = connect('/dev/ttyAMA0', wait_ready=True, baud=57600)
print(" Autopilot Firmware version: %s" % vehicle.version)