A: Hi Matthew,
Thank you for contacting Printerbase.
Canon don't list this as a supported media type however we have had customers say they have been able to print on 300gsm card stock with no problems. Canon only support 300gsm photo paper not cardstock. Cardstock tends to be a lot thicker than photo paper hence why Canon don't support this.
If you wanted a printer that supports 300gsm card stock then you would need to look at one of the professional photo printers. Something like the Canon Pro 10s - https://www.printerbase.co.uk/canon-pixma-pro-10s-a3-colour-inkjet-printer.html
This is currently out of stock however we are expecting stock around the 6th of June.
In regards to compatibility with Mac OS X 10.13 yes I can confirm both machines are compatible.