Agreed J.holoschoenus off your images & description. From looking at PlantNet J.prismatocarpus should have flattened culms, while J.holoschoesnus has terete culms (as is shown here). Agree with you note about the capsules for eliminating J.fockei. I'll confirm this one and change the other ID.
As a general note I usually find that J.holoschoenus (to 75mm) is considerably larger and more robust than J.prismatocarpus & J.fockei (both to 45mm). I find the fomer along creek beds and stream margins, will the other two seem to be more common around pools and inundated grasslands (but that's all very general).
relax, Ciaran, i got my Juncus notes muddled up ( sorry). so i will delete the remarks about height for holoschoenus, which actually apply to the remotiflorus.