Realistically, TIME would also be an issue for Australia ordering any subs other than those already built. Take Singapore's
Invincible-class/Type 218. That project had the initial order (for two) placed back in 2013 with the lead sub launched in 2019 and undergoing sea trials with delivery expected some time this year IIRC.
Typically the contracting phase for a military acquisition can take two years or more. For a platform as complex as a sub, especially one that needed changes made to accommodate Australian and/or US kit, I could easily see that taking even longer. All of this would need to be resolved before construction could commence. Also worth noting is that an established shipyard and workforce would need to be available. In the hypothetical scenario where Australia did decide to order some Type 218 subs from TKMS, the Kiel yard might have capacity to fulfill such an order, with delivery falling some time after the 2024 handover of the additional Invincible-class subs Singapore ordered in 2017. Now to this into some perspective, Singapore added onto the initial order placed in 2013, so the sub programme had already been underway (
) for four years, and the anticipated delivery for the 'extra' subs was/is ~seven years after they were ordered. That introduces the potential for deliveries to Australia to not be able to occur until ~2028, and that is if Australia ordered Type 218 subs that are configured exactly the same as those currently under construction for Singapore.
Aside from the enormous costs that would be associated with ordering an interim sub fleet, there would also be problems getting the number of trained submariners needed to operate all the subs planned for the RAN, plus for the interim order. I certainly cannot see it being managed within the next five years, there is just insufficient time.