I am not sure that can be done. For some devices, the free backups only apply to “Storage saver” quality, which means decreasing resolution and quality, which obviously couldn’t be done here.
Secondly, there is some file size limit. I think 50MB/image, but you could split the files, if it worked in the first place.

Recently I’ve read a comment here on Lemmy about someone who keeps an old pixel at home on a charger, and runs Syncthing on it to sync pictures to the phone so they will get automatically uploaded.
If you were to go that way, you could obfuscate your images before copying to the syncthing shared folder. If you were to encrypt it, be sure to only encrypt the content of the images, to keep any headers and such so that google still sees it as an image. You may try with encrypting the whole file, you’ll have to try it out if that works.

Other things that may be interesting:

• several years ago (2-3?) I’ve heard of a fork of the Simple Gallery app (the orange gallery app on fdroid) that if I remember correctly encrypted your photos and uploaded them that way to google photos. In google photos they were “viewable”, but they were just noise
• you mentioned Docker, so I suspect you’re not afraid of selfhosting. If you have the storage, you may like Immich. It’s basically a google photos clone. App on f-droid.

You’re trying to trick Google into giving you paid service. It’s very much different to ad blocks and whatnot. This will cause you mountains of problem, if possible in the first place…

I know it’s not the point, but I do think it would be easier to set up a photo server than to hack around Google.

there are modded google photos apk but idk how much of that can be trusted

The word you’re looking for is steganography

I just buy a pixel 1 for 50 bucks. Then I use that to upload my current photos. Works flawlessly

That’s a recipe for data wipeout.