Does Peach's castle just get remodelled/renovated often or does she, and by extension, the Mushroom Kingdom Royal Family, own multiple castles?
With how much Peach's castle and its appearance has changed so much between game entires, I've attempted to explain why via 2 options:
Option 1: The castle gets rebuilt, renovated and remodelled often due to:
Structurally rebuild from Bowser's attacks and more, as seen in Super Mario RPG, Paper Mario, Super Mario Maker 2, etc. That and to accommodate various Karting, Sports, and Party events the gang often participate in.
Option 2: Peach, and by extension, the Mushroom Kingdom Royal Family, own multiple castles.
Ok fine I'll admit, this one isn't really that much of a theory and more my personal headcanon, which I know is kind of crappy, but I still thought it was kinda interesting.