You are correct; my assumption that one multiplier will work for the whole 1 to max journey is perhaps optimistic. There are so many sources of XP that it is impossible to prime the player's level to be in tune with their questing. Also, anything beyond class stories' levelling is all over the place and likely with many, many complications. E.g. later expansion, the content was designed with fast levelling in mind.
This is why I think having a single, simple multiplier is probably the best. It just needs to prevent over levelling. Since it is an optional item, if a player feels they fell too far behind - they can simply take it off and run some FPs to catch up.