Firstly, while there’s always a risk that a firmware update may fail or cause issues, it is a VERY rare scenario that any data will be damaged, and you should expect firmware updates to work fine. Backups are always a good idea though!
Google Photos will work, but you may need to pay extra for your entire library and you will have to ENSURE you have the “Original Quality” option turned on first, or they might delete the Lightfield metadata of Leia Image Format images (it won’t affect SBS or Quad Lightfield images though).
You could always manually upload to Google Drive as well, but again there is a slight risk that they may compress photos in certain cases if not configured properly.
I’ve heard VERY good things about Titanium Backup and Sync.com for Android.
I personally just offload all my content at regular intervals onto my computer and then push it to offline loose hard drives for long-term backup and storage (and only keep my best content on the Pictures folder of my Lume Pad) but I know that’s a bit of a hassle and most users probably won’t want to do that. So I’d say the easiest way is a properly configured Google Photos.