A lot of the assets for Aurum Vale and the rest of the other dungeons were already in the game pretty much since release with some of them close to being finished, they were even intended to be non instanced, they merely finished them or rearranged them as the game went along. There were also other dungeons like The Fesse, Hengr’s Crucible, The Ayerie and Peacegarden Croft that were never implemented but instead their assets were used to make Dzemael and Totorak.
Ifrit was probably also planned to be implemented closely after release alongside Titan, you can even catch a glimpse of Titan in action in the very first XIV E3 trailer, which indicates primal fights were far enough into development in some way or another. It does however align with what you mention about the Garuda fight being developed closer with ARR in mind seeing how Garuda was introduced well into the Meteor storyline and that was all Yoshida's idea, although a Garuda lookalike is featured in the original CE and she was mentioned in one of the original main storyline quests.
Stuff like Hamlet was also planned since release, you can look back in the old Lodestone and see it being listed as a promised feature before the team reshuffle. A lot of the stuff implemented up until 1.23 was most likely stuff that was halfway done or close to finished before ARR was even imagined, remember 1.0 was in development for about 4 to 5 years but got heavily rushed to compete with Cataclysm, they were just given enough time to polish it and implement it while working on ARR behind the scenes. Makes more sense to use existing concepts, code and assets intended for an existing engine and finish them than backporting assets and code from the new engine to the old one, especially considering the old assets had higher polygon counts and overall more quality and got downscaled for ARR.