You're not alone in your code mirror confusion. This worked for me;
Go to Control panel, plugins, get more plugins, open library (while online), scroll down and you'll see the basic code mirror plugin, which I assume you've installed already, and saved and reloaded your wiki. If you haven't done that, do it first.
Below the base code mirror plugin in that library list are additional code mirror 'extras' such as autocomplete, close brackets, close tags, and search and replace. If you install those and any others needed, then save and reload you should see a new tab in your control panel. Look in Control panel, settings, Code Mirror.
Use Control-space to autocomplete when in edit mode.
Try this as a font-family if that field is blank and you don't like monospace;
-apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol"
That's the limit of my knowledge hope it helps you.