My first visit to the Island of Bardsey was not an unqualified success, with us ending up a few kilometres away from where we intended on the return crossing. I've taken a keen interest in any information I can find on the tidal streams in Bardsey Sound since then. A few forum posts seemed to agree with my hunch that the guidebook information may not be accurate. That led to me scouring all sorts of data sources in what has become something of an obsessive time sink over the past few months. Anyone interested in the tedious details can find them here.
I still don't have absolutely clear answers on the slack water times in Bardsey Sound. As far as I can make out, no one has carried out reliable measurements. The uneven sea floor and fast tides make placing instruments difficult. A collaboration between Bangor University and a tidal energy company were planning to take some measurements pre-Covid, but I've not been able to find out if that ever happened.
However, it turns out that the tidal streams have been measured, at least approximately, by Bangor University researchers Alan Elliot, David Bowers and B. Jones and that they've been simulated in fine detail by another Bangor researcher, Matt Lewis. Despite both of them having left the university, both David Bowers and Matt Lewis were extremely helpful in sharing information with me.
Looking at these data and a range of other sources, I've pulled together my best guess into a tidal stream atlas here. My best estimate of slack water times are Liverpool -0:15/+6:12.
I hope this information is useful, even though I don't think it's robust enough to rely on. Hopefully, I'll make it back to end of the Llyn peninsula some time soon to try it out...