Thanks for the answer : downtime isn't a problem since I am the only one using the server for now, and I still have leftovers from a previous setup (stremio, local music) for the days I will be without the server.
You are absolutely right, I haven't checked the backups yet, but I will make sure to do it. I was thinking to wipe radarr or sonarr, reinstall the container and restore from their manual backups, to see if everything works. For Immich is probably safer to start from scratch, I don't have that many photos backed-up anyway and I heard it might not always work.
Nice to know I am not alone! And I only recently started with linux, so I guess it's not really something that goes away with time π
I need to take a look at an immutable OS sooner or later, although I feel I would be annoyed for not being able to modify stuff for my liking (probably I will never be satisfied in any case)