Hence what is the best way to organise catalogs and the relative photos in order to easily being able to work on them when needed?

For the Lightroom catalog, set a backup to a Network Storage location every time you close the program. Setup a backup of the images happens during the import of images to the same backup location which the catalog backup is backed up to. If an image goes missing, you able to copy it from the network storage back into place on local storage. This way you can have one catalog for all your images and when your local drive starts to fill up, you are able to use Lightroom to move the files to the elsewhere without losing the link between the edits and the files.

If Lightroom states it's missing a file, you may have to move it manually back to its local location or update Lightroom to point to the backup. Lightroom does non-destructive edits so the edits are stored in the catalog. As long as the preferences are setup to backup the catalog, all your post processing information is saved. When you export a photo from Lightroom, that is when you will store that in a location of your choice. Lightroom will not make a backup of the exported photo as you would be able to export the photo again if need be. You will have to setup your import settings to include a backup copy location and it will copy everything you are importing via Lightroom, Images, videos, Etc.

From a Backup standpoint:
Individual drives can fail or lose data, might be better to look at a Cloud solution or a NAS (Network Addressable Storage) with Raid 1 or 5 or something similar, if the files are that important. When dealing with backups, you always need to understand there is some level of risk and/or cost with each kind of backup and pick the right one for you to use.

a notes for using network storage:
If you are using a MAC, you can look at mounting the share as NFS or if it's Windows, must map a network drive and Lightroom should see the location.