The problem is definitely gpsd that occupies dev/ttyUSB0 on the startup. I used the information on this
to modify the initial launch of the process. I opened /etc/default/gpsd and I just changed the USBAUTO option to "false", full script below:
# Start the gpsd daemon automatically at boot time
# Use USB hotplugging to add new USB devices automatically to the daemon
# Devices gpsd should collect to at boot time.
# They need to be read/writeable, either by user gpsd or the group dialout.
# Other options you want to pass to gpsd
Now when I start Ekos on my Raspberry PI 4, Sesto Senso 2 is able to connect without any problem.
Last night there was a very cloudy sky and I couldn't do any test but I run the calibration of the focuser and it worked perfectly. I will give you some extra news when I'll get a chance to take some pictures.