Welcome to the forums.
About the USB connection problem, if B. Diddy's solution didn't work you could try changing the USB configuration in the Developer Options, select MTP and restart the phone.
About the SD. In most recent phones, like Moto devices, the only way to move apps to SD was formatting the card as adoptable storage. Unfortunatey this option is not compatible with file based encryption, the system used by the X4 among other devices. In general adoptable storage is available in less and less devices due to security and performance issues and native settings like in some Samsung devices are not so popular.
It's strange that system takes so much storage in your phone. What model do you have? In all Moto X4 models that I've seen or heard about there are at least 18GB free for user with system and preinstalled apps taking about 13-14 GB.
