Seems a very inefficient design, from the onset, to require a redownload of all of the data to have a backup file.
Why not have FTXCentral itself generate the backup file from the installed data right on the user's PC? FTXC3 installs add-on by downloading content from servers, then zips/stores a backup sourced from the installed data, which FTXC can use to re-install later. Content only gets downloaded once. Any subsequent reinstalls occur with FTX Central using the local backup as the source instead of the server. And include an option to generate a backup from an installed scenery on-demand so sceneries can be re-archived after an update is applied as well as allowing users of FSS-sourced installations to save an FTX Central v3-compatible backup of each scenery.