Transfer iOS photos directly to external SSD, bypassing Mac

Connect the USB SSD to the iPhone's USB-C port and export images to its Files.app folder. With old iPhones with Lightning port it might be more difficult to find such SSDs and adapters. Take care not to use a SSD drive that requires more power than the iPhone supports.

old_disk wrote: Image Capture native MacOS app is what I was looking for! Solved!

Image Capture is great, but of course you have to use the Mac, and files travel through the Mac--so it's not what you asked about. But I hope it works for you!

Plugging in a drive, even with a Lightning port, is easier with the newer iOSs, and iOS to USBc adapters are available. For instance, I saw this on Amazon

for $7. I haven't used this, and this is just the first thing I saw…

But I've got to say, the USBc port has made a great improvement in my use of my phone! Transferring selected pictures is just Share>Save to Files and navigate to the drive. So simple!

The Mac iCloud Photos database has lots of information (including face names, and cropping and editing steps) that can't be directly exported, at all. So the only way to preserve all of the information in Photos is to copy the entire Photos Library Package to an external drive. Time Machine can use incremental backups to keep the Library copies current. Saving pictures out of the Library and handling them as individual files loses lots of information and flexibility that Photos offers.

You need to keep at least 10% of your Mac's internal drive free. I've seen trouble on my 1TB MacAir when I went below 15%, 150 GB, free. This is real. You may need to offload less-used files to an external drive. You don't say if you're using a desktop or a MacBook, but iCloud's Optimize Storage can help a lot with saving space. The downside is backing up pictures that aren't actually on your machine. Here is a solution for that:

Backup iCloud Photos from an Optimized Mac - Apple Community

Also, to see some about how Photos' Image Management works differently than Finder's File Management, see these:

Basic Concepts and Terms About Photos - Apple Community

and

Organizing Photos with Folders & Albums - Apple Community