IMO the USA would use tactical Nukes/conventional Nukes if the US home territory was under attack and that conventional defence systems were failing/likely to fail.
IMO the same applies to Russia or even any Nuke armed country with lesser Nuke abilities such as Israel, North Korea , Pakistan etc etc.
And of course, the rationale of AUKUS is that not alone are the subs Nuke propelled, they will have the ability to carry either US or UK Nukes capable of complementing US first or second strike capabilities on either China, Russia or China-Russia combined. (Our Claytons' Nuke Option)