Encryption makes the backup task more difficult. That's why I avoid encryption except where I really have the need for it(which was never case for me so far, and thus I won't be helpful on the topic due to the lack of experience with it). You probably want to encrypt your backup space too. You need to get aquainted with the tools to mount the encrypted storage from within your running os and from a preferred rescue system.

Since you're a beginner, a CloneZilla can be a fallback solution until you're famillar enough with your linux os. With CloneZilla you can save and restore the os partition without knowing very much about linux.

For an easy start you may take a usb disk and put your data there. A more advanced and safe approach can be to have a networked device that connects via network to your computer and backups the updated data regularly. (rsnapshot can be used with both variants).

There are lots of backup tools. A simple backup tool is the mentioned rsnaphot. I would recommend it too. It's a file based backup in contrast to the image based backup of clonezilla. It's not primarily targeted for full system restores, but that can be done without problem too. If your system crashes completely, you can take the following steps to recover an os installation by only having the files:

Full system restore to an empty disk
These are quite some steps to learn the commands if you never did it. But once you got that, restoration is easy. For me with many years of linux experience this had become childplay for me. I did it hundreds of times with very different systems and in contrast to windows, where this is just not possible: There maybe challenging advanced linux setups, but 99,9% are solvable and most of them with ease, when you have the basic knowledge.

... and of course one can just examine the problem using a rescue system and fix it without the need for a full restore.

Some cases I experienced which required fixing it from outside the running os:
Yes. And for him and many others in the field of the OSS-Community it's a complete showstopper. It's absolutely inacceptable for those to put work into something where lawyers could come and pry the work out of the hands of the community.

There are tales from the past about the enormous memory hunger of zfs. Those tales belong to the lands of fairy tales and myths. But at the core there is a grain of truth. btrfs is more resource efficient than zfs. I read that a system with 1 GB should be an adequate minimum for use with zfs. If you have a new system with 8 GB of ram or more, this won't be a limiting factor. And again: If you really want to use deduplication: Carefully read the documentation before you decide to use it! And as already written: the same goes for btrfs!