iphone – Backing up huge icloud photo library to external HDD

I have a large icloud photo library of about 30,000 images and videos – lots of baby photos of my kids and really irreplacable stuff like that. I’m getting concerned that icloud is the only place I have the full resolution images stored and I want to download the entire library to an external HDD (preferred), or a second cloud service (less preferable) to ensure I have a second copy. But 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 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.

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 a 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? Don’t mind manually adding future pics to the backup.

Thanks