backing up full resolution photos to an external drive and turning off iCloud for photos on all devices

Or, maybe the easiest way would be to make a new Photos Library, set it as your System Library, connect it to iCloud, and then let it automatically load all your pictures and albums and folders from iCloud. Then it's done. If your external hard drive is formatted in either APFS format or Mac OS Extended (Journaled) format, the new Library could be created on your external drive, and you don't have to move anything.

Photos for Sonoma should show duplicates in the Duplicate folder. You can open up the Info window (command-i) and see the resolution (actually, the number of pixels) of each as you click back and forth. This may not be indicated in the file size-- you can have higher resolution that is more compressed. More pixels is better than more bytes.

DPI, the dots per inch, just depends on how you print it. More dots (pixels) is better. We sometimes say that 2000x1000 is better resolution than 1000x500, but we mean if printed the same size.

If you ask Photos to "Merge 2 items," it will pick the one with the higher resolution and combine keywords and other metadata. If they're equal but have different metadata, then it may be a bit of a crapshoot. Once it decides on which is better, then Photos will delete the "less better" one and then copy the remaining "better" one into the albums of the other. Deletions always got to Recently Deleted, so you have a month to change your mind.

There are few other duplicate finders for Photos-- you have to be careful of 3rd party apps really screwing up a Photos database. I've used  PowerPhotos ($30) when I've been impatient for Photos to find duplicates, and it will show resolution data for two pictures together. It also helps with a number of jobs with more then one Library.

If you can, it's best to "Download Originals" on your Mac (or external) drive as you've done. Way better for backups!

Seems fine. Your first problem will be #1-- Photos may not download pictures in full resolution unless it thinks it needs them. Photos needs full resolution pictures when it edits or prints. You may be able to force a download by making of slideshow of all your pictures. Another method would be to print them all (not to paper!) to a pdf. You can choose to print; fill the page; print; then print to pdf. This will make a huge file-- you can put it on your external drive and erase it later.

By the way, have you edited your pictures, added comments and keywords, or are all your pictures in their original form?

Great! Let us know how it goes...