GCam requires the Google Photos app to be installed to load previews.

One way to fix this without installing it is to use GCamPhotosPreview. I use it myself and it works as expected

There’s an open issue on the Github repo, but there hasn’t been much progress

Kaiteki aspires to be just that, but it’s currently in alpha

Nice to see a balanced opinion, this whole facebook/meta discussion has been pretty virulent at times

like many have already said, a custom ROM is a pretty good option. Aside from LineageOS, I’d recommend DivestOS which is a fork with some security and privacy enhancements