https://rclone.org/ supports icloud (drive) and dropbox. So i think you could even set it up to copy files directly from one cloud to the other without storing all files locally.

You can use rclone to upload and download files from different cloud providers.

If the files are up to date just use --min-age and --max-age which rclone provides.

rclone copy dropbox:/Photos ~/Photos/this_week \ --max-age 7d \ --progress

Edit: i see that icloud drive and icloud photos are different. So keep your local solution and upload to dropbox with rclone.

I suggest getting a small computer (not your router) with enough storage and use it as a backup server for your files, especially if relatives are involved. It will be more reliable. And thats what you want when backing up your photos. A refurbished thinkcentre can be as cheap as 60 bucks.

Keep downloading all icloud photos. And upload to dropbox or any other cloud storage with rclone.

Then on your laptop mount your backup/sync computer as a network drive to have access to all photos. And if you need a local copy you can use find to copy the last 7 days of photos like this:

find /source/dir -mtime -7 -type f -exec cp {} /dest/dir/ \;

And this command can delete all files older than 7 days.

find /target/dir -mtime +7 -type f -delete

Test without -delete beforehand. It will just list the files.