My own preference is to merge Backup (as opposed to Sync) with sharing. If the backup service doesn’t provide some obvious value to the customer, they are unlikely to keep it up. If they get something more (like backup + easy sharing/using) they are more likely to keep adding to the backup site.
For the cloud, I focus on how they want to consume collection daily. Do they have any digital assistance devices with screens? (Google Home, Amazon Echo). Do they want a daily ‘these are your photos from today in the past’ prompt? Google, Amazon, OneDrive, and some others do this very well. Apple may, but I don’t know. SmugMug is pretty awesome for this if they use Lightroom Classic to keep it updated with new photos and want to sync back comments family members may add. Lightroom CC’s web interface is also good, though a little spendy.
For pure backup, Backblaze is excellent, but they won’t be interacting with the files there.