There is no disadvantage to booking every trip as a (or series of) one-way journey, and I almost always do this. Especially if dates are not set.

In some rare cases, there might be some benefit to having a series of flights (eh all in one day) on one PNR for ‘protection’ against disruption.

FlexiDate is the only fare category that is refundable, so that is also a criteria for me if there is some uncertainty or I want flexibility.