I have been using rclone for years, it's great. https://rclone.org/iclouddrive/

I have Synology server at home. Once in a month when I put my phone on the charger I open up Synology Photos app on my phone to backup all the media to that. I intentionally don't allow it run in the background because I don't want to drain the battery. This way I have full control and possession of my media. As a second(third) wave of defense I also have my Synology data automatically backup to Backblaze B2 cloud which is quite cheap.

> I want to programmatically backup my iCloud picture library to a second cloud.

Fastest is to get a mac and enable offline storage for Photos.app, then backup the resulting folder.

> what is the best way to bulk download your iCloud image library?

The best way is to use Photos.app on macOS and export the photos (full resolution). This is what I did a while back

I have the Google Photos, One Drive (part of Office365) and Amazon Drive (free with Amazon Prime) apps on my iPhone. Photos automatically get synced to iCloud, Microsoft, Amazon and Google. Dropbox also has photo sync capabilities from your phone.

The free Amazon Drive only allows you to back up 5GB worth of videos. But unlimited photos

You do have to remember to open the apps occasionally.

You can download all of the originals using the Photos app on the Mac or on Windows

https://support.apple.com/en-us/108994

I don’t have my personal computer up and running enough these days to make this feasible. But, once you download photos to your computer, you can also use BackBlaze.