Try cobalt.tools. It does what you want to do inside a browser tab (although I only used it on desktop so far), but it might work with an IPhone as well. You can convert YouTube links into either mp3, ogg, wave and opus files (with various bitrates), or mp4 or webm video files (with various resulutions). I use it constantly.
As long as you have multiple backups of your data, you shouldn’t be concerned. HDDs as well as SDDs can potentially fail at any given time.
The key is to have more than one backup. You shouldn#t rely on only one backup alone.