Online backup strategy or software for computer and external drive

I'm looking for an online backup strategy for my photos but there's something that none of the systems I've looked at addresses...

I keep my most recent photos on my mac and move them off to an external drive from time to time. I can't keep them all on my computer because I don't have enough space.

Most systems I've looked at can backup both the computer and the external drive. But when I move photos to the drive they will be removed from the computer backup and re-uploaded to the drive backup. This is not only a waste of bandwidth in re-uploading but I'll have to leave the drive plugged in until they are done uploading again. There could even be a time when they are not backed up - after they are 'deleted' from the original location and before they are uploaded again from the drive.

Can anyone suggest a system that handles this or a better way for me to manage my photos?

My photos are all in Lightroom, there are about 500gb so far, I guess I'm adding a few gb per week.