Dare you look at Storage settings to see how much of the SSD is now swallowed up by System Data? This should allow your Data volume a maximum of 228 GB, less any space required by the VM volume. Although installation of a macOS upgrade or update will require substantial additional space, once that’s complete the space taken by the System volume and its snapshot should fall to little more than 12 GB. So an update-style upgrade, as you should get when going from macOS 15.7 to 26.0, should require a much smaller download, a faster install, and less space to install the new version of macOS. Wait a day or two after upgradingInstalling a macOS upgrade also changes files on your Data volume, and may retain temporary support files.