I'm told (although I haven't found anything official on this) that non-toileted trains are currently only allowed on journeys with up to 30 minutes in the schedule to a toileted station, which is why single 153/9s occasionally appear on the Coryton/Penarth/Caerphilly route, and the Coryton/City Line and Cardiff Bay shuttles pre-June 2024.
(CPH>CDQ 17mins,
COY>CDQ 17mins,
PEN>CDF 11mins).
It's not uncommon, however, that in disruption passengers and crew are stranded on the unit at an un-toileted station or in between stations.
But yes, the 398s will take a lot of convincing for me to think they were a good idea for the Taff services. They'd be absolutely perfect on Coryton services - short distance, start-stop, plenty of capacity for the passenger levels etc, and their acceleration would help with resilience in the schedule and increase to 4tph when the Rhiwbina/Birchgrove loop goes in. If the Penarth line were to be electrified, I daresay those COY/PEN/CPH diagrams would be doable with 3-car 398s.