I keep my Photos library on an external SSD and I am wondering, if I should include these 300 GB in my backup routines (Airport Time Capsule, Backblaze and Carbon Copy Cloner).
How about iMovie? Is there a way to get the stuff in the cloud other than dragging ist over?
Definitely have a backup of the SSD on another drive. You could add it to Time Machine preferences if you use Time Machine (and the destination drive is big enough). Time Machine seems to manage Photos backups quite well.
I assume that you are using Photos and therefore all your photos are stored in a single “library” file on the SSD. That makes it a little difficult to have an incremental backup process but you could regularly copy that very large file to another drive. This also probably means that CCC and similar would need to backup the whole file and not just the changes (CCC experts might chip in here…)
As well as this I use 50Gb Bluray disks to keep a backup of the Photos library (they are not re-writable and so I have to burn a new disk every few months but on the plus side they are supposed to
last for decades) . However this means I need to archive older photos to keep the Photos library file size under 50Gb. I use Power Photos for this task - it can move photos to an archive library file.