So I've managed to upgrade by running
plesk installer upgrade PLESK_18_0_18
It's weird that this option wasn't available in the GUI or some command lines though
Is it possible to perform an In-Place upgrade to Plesk Obsidian? I'm currently running Onyx Version 17.8.11 Update #65 and there is no option to update past this point in my "Updates & Upgrades" page