I get where you’re coming from on that, but understand that a sideloaded game (not installed through Google Play) can’t have a sign-in feature unless it operates its own account system and servers to store the data. Which is a big job, and expensive too.
If the game is distributed through Google Play then you can be signed in to Google and your game data will be synced to their servers (I think for free — to the game developer, I mean), but sideloaded content doesn’t have that option.
I completely understand the frustration of losing progress, and the need to have some way of preserving game saves outside of the app install, since Android is so aggressive about sequestering each app’s data and tying it to the app itself.
What might be possible, instead of cloud-based backups, is a feature that lets you export/import a saved-games backup file stored in your Android “Documents” folder, so it’s not linked to the game install.
But even something like that, I’d think would have to be implemented in Ren’Py first (and the app would need to ask for new permissions)… which would mean, even if it existed, Xperiment would also have to upgrade Ren’Py again to make the feature available. Catch-22.