Unable to download (some) photos?

Hi, I am on a quest to update my phone.

Background:

I want to backup all my photos first. I had around 6500 photos on my phone. My phone was completely full. I synced my phone to my mac, and almost 6000 photos were downloaded and are now backed up and deleted from my iPhone.

Problem:

However, it was missing approx 600 photos. Upon much research, I believe that it is because my phone was full, and these 600 have been uploaded to iCloud. They each come up with the loading circle that indicated that is is downloading from iCloud. I have been through every photo and ensured that now that there is space on my phone, and every photo has said that it downloaded-but I can only import about 60 more to my computer. It's driving me crazy, and my phone desperately needs an update (it was unfunctional before I deleted the earlier 6000 photos).

I have logged onto iCloud and am trying to download every photo individually, which is not only a long process but doesn't seem to be working reliably for some reason and I still appear to be missing photos.

I have an iPhone 5 9.3.5. Yesterday I switched the 'Photos and Camera' setting to 'download and keep originals' instead of 'optimise iphone' storage. I have tried to download the photos on both the mac and a pc.

I don't understand why it is so complicated, and would love any advice or ideas please. Thank-you!