I have a large iCloud photo library of about 30,000 irreplaceable images and videos. It is entirely hosted on icloud - I am using the 'optimise iphone storage' setting and have no high res images stored locally on my laptop as the library is too large. I'm getting concerned that iCloud is the only place I have the full resolution images stored and I want to download a backup of the entire library to an external HDD (preferred), or a second cloud service (less preferable) to ensure I have a second copy. I am happy keeping my main library on icloud as it works well for me and allows me to sync all my different devices (iphone, macbook, 2x ipads), I'm just looking to make a second backup copy in case something goes wrong. I don't need to use or access these photos under normal circumstance.
I can't work out how to do this. The only options I seem to be able to find is to either download 1000 images at a time from iCloud to my external HDD (which is near impossible as I can't work out how to select 1000 at once), or download the entire library to my laptop, then backup from there - which I don't have space for on my laptop (photos library is 250GB, laptop entire SSD is 250GB).
I'm not especially concerned about preserving file structure or anything like that, I just want to make sure I have all the images in case anything ever goes wrong with iCloud. What is the most sensible option? I'll pay for 3rd party software if that's the only option, though happy to put in some legwork to do it manually if there is a sensible way to do it. I don't mind manually adding future pics to the backup.