Deleting specific photos from my iPhone that were imported "From My Mac" 6 years ago

Photos that were placed onto an iPhone by syncing them from a Mac or Windows computer (using iTunes or more recently Finder on a Mac) were designed to be deleted from the Mac by syncing with the computer again but without those photos selected to sync. That is described in Sync your photos using the Finder - Apple Support including this excerpt from that support article:

If you don't want the synced photos on your iPhone, iPad, or iPod touch anymore, turn off syncing in the Finder and they'll be removed from your device. You can turn off syncing for some of your photo albums or all of them at once.

There are two 3rd party apps that may allow you to more easily delete the synced photos and also may be able to give you the option to copy them to a computer. Those apps are PhotoSync https://www.photosync-app.com/home

and iMazing https://imazing.com/

See the following excerpts from https://imazing.com/guides/how-to-manage-and-transfer-photos-between-iphone-ipad-and-computer#export

(note that according to their article, they cannot edit the synced photos, but can delete them and I think they can also copy them to a computer)

You can only delete photos from the camera roll (if iCloud Photo Library is disabled) or from From My Mac albums. 

⚠ Important: Synced albums, whether they are managed by iTunes, the Finder, or iMazing, are always listed under the From My Mac section, which is how the iOS/iPadOS Photos app labels them. They cannot be edited on the mobile device itself, a restriction imposed by Apple that iMazing cannot workaround.

That's correct with a slight modification.

You could use any computer and sync and delete all 3737 photos.

Or you could use any computer that still has the original images and delete all and add the 2000 you want. It doesn't have to be the original computer but you do need to have the original photos.

I have used PhotoSync but it has been a long time ago and I don't see anywhere on their website where it says yo can delete the synced photos but they did support that years ago.

I haven't used iMazing but their website seems pretty clear that you can.

You may want to contact them or see if they have a trial option.

"yet if I try to edit or delete anything from before 2019 the options are not even there."

I can no longer test this on my current iPhone, because it is syncing with iCloud Photos, and so I cannot have manually synced photos on the iPhone, but previously it has been possible to duplicate synced photos on the iPhone and thus save a copy to Photos, that can be used like any photo saved to Photos and remain on the iPhone, when you sync again to remove all synced photos. You may want to try use the Share menu to duplicate the synced photos - just the photos that are most important to you - and thus save them in Photos as regular photos.

This was helpful. Merci Léonie!

You're welcome, rachelleL 😊