You can customize the modifier keys (control, option, cmd, caps lock, globe) but only to the other modifier key options (i.e. change ‘control’ to ‘option’) or as an Escape key (which I did map to my caps lock), but that’s it. Nothing for it inside Accessibility
Full Keyboard Access allows you to use keyboard shortcuts to navigate but those options are also predetermined (i.e. use Tab-C to open Control Center). What I can’t do is map keys to control volume, brightness, keyboard brightness, media, etc. Activating Full Keyboard Access also produces a perpetual blue box that highlights what is selected on screen—quite annoying and not worth having one or two keyboard shortcuts.