Kiastien:
namely installing the CH340 USB-SERIAL driver
If it's truly a genuine Arduino Starter Kit, then those suggestions don't apply. Only the Uno derivatives made by 3rd party companies use the CH340. Arduino does not use the CH340 on any of their official boards.
Kiastien:
I am using a usb hub (because I can't do anything otherwise thanks to Apple!) but am not sure if that could be the source of these issues!?
Often we recommend trying a USB 2.0 hub between a USB 3.0 port on the computer and the Arduino when people are having this sort of problem because the USB 3.0 is known to cause problems sometimes. I haven't heard of a hub causing a problem like this, only solving them.