It's not possible to download eternum on Android 12 but is it possible that you're going to make an eternum apk instead of zip?
Just because you can't access the Android/game directory doesn't mean you can't install the game at all. Just skip that step. You may lose your saves, sorry about that, but the new Android version will store the saves on your shared device storage, so this won't be a problem anymore after you made the transition.
I understand that the new installation process is more complicated and that you'd rather have just a single apk to install. However, the maximum size of an apk file is limited to 2 GB. For comparison, the PC version is already 7.5 GB large! There's some magic you can do with media compression, but everything will look & sound worse and worse the more you're trying to cram into that 2 GB box. The transition to this new installation processes had to happen sooner or later, similar games of this size do it too. I hope you understand.