When receiving an invitation I normally import it as ics file into the calendar. But it is not possible to add a category, for example "Journey", "Holiday" and so on.
My suggestion is the possibility to add a category in the Import window (see attached picture) or in the context menu, if the event is already imported.
The best solution would be to make received events editable 😀.
(I know how to workaround this by editing the ics file, but it's tedious).
Best regards