As far as the slicers go, yes, starting with the onset slicer would be a good idea, as it might respond best to the spectral changes of the different phonemes. I would probably listen to the sounds through the real-time version of the onset slicer
fluid.onsetslice~ to hone in on what settings you prefer.
Phonemes can be pretty varied and very short with subtle differences, so a good exercise might be to take some of the audio file and manually slice out the phonemes, just to get a sense of the durations they tend to be and how they flow into and out of each other. That might help you hone in on what you’re looking for!