I can think of one way (which I used for a while to make backups of my daughter’s iCloud photos). But it’s awkward:
Create a new Photos library on your backup device. The easiest way is to hold down Option when launching Photos. Do not change your iCloud configuration - leave it pointing to the original library
Launch Photos with the new library (double-click it, or option-launch the app)
Import photos from the iCloud library. This should (I assume) cause your Mac to download the images (if necessary) and then import them into the new library, on your backup media.
I don’t think this will work. Presumably exactly for this reason (that since you don’t have the iCloud library open, Photos can’t download photos), you can’t import a Photos library that uses iCloud (emphasis mine):
In macOS Monterey or later, you can import photos and videos from another Photos library, as long as the library you want to import from doesn’t have iCloud Photos turned on.
Hrmph. You’re right. I didn’t notice that line. So that’s not really an option either.
If you turn off iCloud Photos, I assume that would cause them to download locally. But if you do that, then a normal backup would work. And it would fail if you don’t have enough local storage to hold the iCloud content.
TBH, I’d like a MBA with more storage. While many people don’t need it, 2TB is small for other’s who do, though who don’t need the power of MBP’s other features and/or like the MBA’s lower weight.
I’m certain Apple limit storage on the MBA to push people onto the MBP if they need >2TB. And I bet the next M5 MBP’s later this year will likely offer a whopping 16TB as an option (for a small fortune, of course!) to match the new M3 Ultra Studio, making the 2TB maximum on the M4 MBA seem a bit (deliberately) limiting.
We’ll see whether Apple think this is enough on a more permanent basis, if the M6 or M7 MBA arrive still with this maximum, I guess.