Good work with Samsung, Rod.
As for your question, you may be conflating two separate issues. Assuming your hot water tank is plumbed conventionally (water from heat pump flows either to your emitters or to the hot tank coil via a diverter valve), then the size of the hot tank is immaterial - DHW is only heated for a few minutes each day so has no effect on heat pump cycling, and in any case the coil volume is tiny (compared to that of a buffer/volumiser tank which are large enough to affect cycle times).
The main reason not to install a PHE on your UFH is that you are introducing a temperature loss - (search for “temperature approach” if the term is unfamiliar) - and will need to operate a higher than necessary LWT to compensate.
Or maybe I’ve just misunderstood your question… 