

Presuming BASH:
https://www.gnu.org/software/bash/manual/bash.html#Quoting
Single quotes are much safer/predictable because:
Enclosing characters in single quotes (‘
'’) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.
Not to say they are always tye right coice though.






You’ve made a great start. How much further you go depends on your needs and threat model.
Rsync is ok as a start but there are dedicated backup tools that will give you access to your files at any point in time. This is important if you delete a file and later need it back after an rsync has already run and deleted it remotely too.
Rsync will not encrypt your backups. If a burglar takes your Pi will they have easy access to all your files too?
With rsync+ssh you are also vulnerable to either yourself (more likely) or a rogue process on your own machine deleting the files over ssh (e.g. ransomware attack).
The answer to that is append-only backups. A backup tool like restic has a backend you can host that does exactly this:
https://github.com/restic/rest-server
Take a look at restic to see what else you gain from switching to a dedicated tool versus rsync:
https://github.com/restic/restic#design-principles
There are others too such as borg.