Hi........Im trying to clarify if I eat less than a kazayis or reveit of liquid initially, could I still eat such an amount at some point during the meal (in 4 or 2 minitues respcively) and then say a brocha achrona? Similarly if I have a bite of cake initially but then 10 minutes later have a kazayit of case, would that require a brocha achrona? Thanks!

I understand that you ate bread in that meal, so there is no reason for you to need any kind of bracha achrona, aside from the regular Birkat HaMazon bentching.