I've read on reddit that Google still provides unlimited photos backup if you own Pixel 5a and lower. On rooted Android you can fake that and get free backup storage, but who wants to give pictures to Google?! I was thinking if it's possible or done by someone to create an environment on Docker by faking device info to access that free storage and backup rclone crypted files as images (by .jpeg fake extension or something)?
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: