"So if Astute herself can survive until the first AUKUS boat commissions, the remaining Astutes will be relatively youthful and an 18 month interval between delivery of subsequent boats shouldn't be a problem at all -"
Yes - the Trafalgars started to decommission end 2009 and 16 years later there's still one in service. The first Astute commissioned in 2010 and there are still two to come so you can expect significant overlap for many years with SSN-A