I’m using Glyphs Mini 2, and it is missing several key punctuation symbols; namely quotation marks ’ " and also colons and brackets. Is this one of the limitations of the Mini version, or is it a problem? If so, how can I fix / work around this?
You can add many more glyphs: Adding glyphs to your font | Glyphs
Have you tried Cmd+Shift+G to generate a glyph? Otherwise in the top-left, you can find collapsible menus that contain all sorts of symbols. Right click on those, click generate in the popup, voilà.
(Sorry if this is not the case in Glyphs Mini 2, just guessing)