Sorry I didn't see this sooner, but what app could I use to edit and rename the app's file?
Viewing post in Gacha Luminals Official Download! comments
Apktool M is easy to use, just change the name of the package a bit(replace or add a letter) and it's done, like i said before, this doesn't apply to only Gacha Luminal, it should solve the issue of any Gacha club mod that gives the message "package is invalid" and "do you want to install an update to this existing application?" when trying to install it.
It shouldn't take too long to do this, let me know if it worked.