Excellent! Thank you very much. I’ll try if this option is less laborious than the solution I came up with.
For the record, and perhaps as a useful tip for any future readers — in the meantime, I’ve solved my perdicament by:
* Downloading the free version of Admin Menu Editor
* Changing the required capability for the (sub)items to something only admin has (in this case switch_themes).
So: edit_theme_options is still the capability rewarded to Editor through URE, and it’s required default for Menu and Widgets. But, after changing in AME, it is no longer sufficient for Customizer and Editor, which now require switch_themes capability.