Calamares already supports this. You can specify custom mount points in the manual partitioning setup. For any mount point, you can specify whether the disk should be formatted or not so it is pretty easy to keep your data intact.
In Garuda Linux, @home is also a separate subvolume by default. You can already do another installation and leave the existing subvolumes intact, no separate partitions or disks needed. See this topic, for example:
In my opinion a separate home subvolume is way better than a separate partition; I think we should leave the default behavior of the installer how it is.