I don't follow your reasoning ... "need more space" is never a good reason for backups ... backups should be made regularly and automatically whether you need more space or not ...
So let me ask for a clarification ... do you want to move photos from one disk to another because you need more space? Or do you really want to make backups of everything?
thx for your input.
First: Yes, i have already backups (the raws). The point is, that i use at the moment my internal ssd drive for the actual catalogue, because of the speed.
Second: Now i wish to backup all the photos with all the settings, metadates, librarys etc. to 2 external drives. Reason for that is, that LR should be able to open from this new external drives and that i could erase all the data from the internal ssd.
You seem to be using the word backup in a way that I don't use it. What you are describing is not a backup in my terminology, it is simply moving the photos.
The best way to move the photos is to use your operating system, copy the photos (the entire hierarchy of photos, unchanged) to the new disk, then in Lightroom use the Update Folder Location method as described here Adobe Lightroom - Find moved or missing files and folders (scroll down to the section entitled "Updating Folder Location"). Once that has been successfully done, you can delete the photos from your original HD.
This does not make a backup. If you want a second copy of everything on another disk, then this would be a backup, this has to be done via your operating system or third party backup software.