Google Photos has a feature for exactly this on Android. It's called "Locked Folder". It is not synced, and accessing it requires your screen lock code or biometric authentication. You can choose to save photos directly into the locked folder as you take them, in which case they are never uploaded.

On Android, I use Open Camera app with a custom save location that by default doesn't backup photos. It prevents going into DCIM / camera roll.

Go into more camera controls , enable Storage Access Framework, select custom directory.

If you're concerned about your privacy I'd advise against just trying to disable uploading but still giving your photos to apps you don't trust, primarily because of possible future implementation of client-side scanning(they are making steps to mandate that for messengers in the EU atm, for example).

AFAIK android phones upload all sorts of data to google by default, my recommendation is to install a custom ROM (like lineageOS), that will rid you of a lot of these privacy violators without dealing with them individually. If you don't want to do that, avoid google services/apps entirely for anything that you don't trust them with. I'm not familiar with google photos(never used it for (now) obvious reasons), and I don't know what features you need, so I can't really give recommendations on that specifically.

Turn off Google's auto upload. The entire photos thing has long been a way to get images to train ML, and since they actively use these images you're not in a position of power here. i.e. you're doing Google a favour by using the auto-upload.

Either find a service that you're paying for, or if you want to trust a big corp maybe go with Apple. Not perfect, but best of the lot from what I can see. All of the logic is on-device, so they're less inclined to peek at your photos. There was a big hubbub a while ago when they tried to do unsafe photo detection on-device, but they backtracked.

Apple does same exact scanning of iCloud photos and ban iCloud accounts with the same glee as well, so I'm not sure why do you think recommending another megacorp will help the OP (outside of buying into Apples marketing schtick).

I disabled iCloud Photos sync some time ago, and even though there are some pain points, it's surprisingly easy to work without it. Just do a manual direct sync every so often. Run borg or bupstash to encrypt and backup the copied photos to any location.

don't use google photos at all. And probably don't have pictures that are even remotely questionable on your phone, even if it's for legitimate reasons.

That overlooks the reality of life. To many people, smartphones are the only camera they have.

That's fine, and it sucks that often there is not really a choice in which camera software to use either. It doesn't help that by choosing Android or IOS you almost inevitably get hooked into the clouds of Google and Apple.

I deliberately bought a Pixel 6 in order to install GrapheneOS (a degoogled Android) and leave out all links to Google except for the Play store for a handful of apps pushed by governments and banks. The built-in camera app works fine, and OpenCamera is a great alternative too now that it supports all cameras of the Pixel 6.

This is the correct answer. Use GrapheneOS and synchronize your photos to your NAS. Nobody needs to use Google or Apple "cloud" for photos.

Do not resume it.

I think until Google Photos implement selective upload, we as a user have no options.