This is the wrong comparison. Vendor security patch is something else.
Full “rules” in Post #2
This is a new problem because this is an approximation, it is possible to be anywhere in the range 1st to 5th.
Edit, you do say that you established your current /e/ build has Android SPL 1st May.
Without more exact info of the original Fairphone build you are likely best to wait a month. All data will be lost when you lock at that point.
The true Android SPL on a device is found from Settings > About phone > tap Android version.