How to assign a backup drive during the install?

There's nothing special about it. It's only a question of mount points. I have a similar set up to yours but with 3 HDD not 1 SSD and two HDD - but still, 3 total drives. One has the OS, the other has /home and third is mounted as /backup. But you can give it whatever path in the file system you choose to. I use the netinstall and during the process of going through the install you'll have the option to manually configure how to partition which drives and what to mount where (one of yours will be mounted as /home for example.) I imagine that the graphical installer has something similar. But once it's all done you use whatever software you decide on and start doing backups.

Oh, so I create a mount point anywhere I want, and name in anyway I want? Cool, thanks :)

I followed this guide (https://libreboot.org/docs/gnulinux/encrypted_debian.html) successfully with a single hard drive.

But now I'm trying to install on 3 drives, and still have them encrypted.

To my understanding, I can lvm and encrypt each one of them.
But I just want to keep the /, /home and /backup on their own disk, not logical volume, for I want to be able to reinstall with only changing the disk that holds /.

Should I forget LVM and strictly focus on LUKS?
This seems to be a better tip : https://askubuntu.com/questions/511085/installing-ubuntu-across-multiple-hard-drives

but the problem remains.
I should be able to assign a disk to a part of the filesystem, but I'm not sure about how to encrypt it all.

OK, after a bit more research, I think I get it but I'm still not sure.

Goal is to get all theses 3 disks encrypted this way:
1 disk (so 1 volume group) with several logical volumes (/boot, /, /home etc.)
1 disk as /data (so 1 volume group for 1 logical volume)
1 disk as /backup (so 1 volume group for 1 logical volume)

That's for LVM.
I just don't have experience with creating mounting points with random names (data, backup).

Now what about LUKS? Since I can set up each drive to be encrypted (that's 3 volume groups over 3 different drives), does that also means one encryption passphrase? I wonder because it's encryption over different physical volumes.

I hope it does. More reading, less guesses, I suppose.

I don't even know if its LVM over LUKS (one passphrase only) or the opposite (one passphrase per volume). I'll read more about that.

Oh, and the idea is to be able to reinstall the main disk without touching neither the data nor backup disk, and leave those encrypted. Is that even possible?