You will need to to install the upgrade advice from the News app or from this discussion: Fairphone 4 update to iodéOS 7.4 but can replace 7.4 downloads with 7.5 (as your last thought suggests). What I mean is that a full data reset is needed before the upgrade will succeed.
So while there is no need to upgrade to 7.4 and then to 7.5, it still does require using a terminal and manually upgrading to 7.5 via Recovery, or instead performing a full reinstallation of 7.5. Sorry to not bring you any better news.