I think the deciding factor was the fact French subs use low enriched uranium that must be refueled every ten years which requires a nuclear industry that Australia doesn't have, whereas American subs use weapons grade uranium that lasts the life of the sub. Presumably they decided it was more sensible to go with a clean sheet design instead of putting a British reactor in a French hull.
It seems it was announced just last week that Australia will purchase three Virginia class subs as an interim until the new design is ready, which will be the same used by the Royal Navy and will reportedly have VLS tubes, something the Barracudas lack (possibly another reason they chose a different design).
The future British attack submarine, the SSNR, will feature a vertical launch system (VLS). This is a first for the Royal Navy and will increase interoperability with American allies. This can be seen as a smart move, as the Ukraine War reinforces the importance of supply chains.