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How to Get Android App Updates Directly From the Source
Otanium is a tool that allows you to install and update open source apps directly from their release pages. It's basically just like combining all the different places you can get open source software for Android, like F-droid on steroids by the sounds of it. I've got nothing for your third discoveries, but I've got a message on thread that I started and loads of people reply to it with their discoveries. And so I'll put a link to the thread so you can see everyone's suggestions.