Photos full-res image upload from iPhone to iMac does not work

I have a fully-updated late 2009 iMac running Sierra. In the process now of updating to Mojave. I am trying to sync my pictures from my iPhone XR to my computer via a lightening cable. The photos seldom show up in the Photos app on my iMac, even when I've reset iTunes, attempt to use Image capture, restart, etc.

I have an iCloud account that backs up my iPhone pictures. That's all I want it to do-- I don't want to keep my entire photos library on there-- I have 4 TB of storage on my iMac. When I try to manually download photos, the the files are compressed. Because I don't sync my entire Photos library, there is no option to import iCloud photos through the photos app-- they do not show up.

When I do get the photos from my iPhone to show up in the Photos app, it runs into an error (9307) upon syncing.

I think Apple is trying to get me to buy a ton of iCloud storage. I upgraded the storage on this iMac to avoid this need. How should I approach this issue?