A few of mine :

• AndOTP : A good app for OTP that actually let's you backup your keys and move them easily and securely to a new phone
• Blokada : A system wide adblocker for Android. 4000000 ads blocked so far on my phone
• YouTube Vanced: Not really a "hidden" gem but a cool app nonetheless. I wish people knew you could get an ad free YouTube experience on mobile. It also includes sponsor block so you really get straight to content. Also let's you get a YT popup player and shutdown the screen with the video still playing.
• F-Droid an Android open source app market. Often includes apps that are not allowed on Google market.

To some extent I think you can do most of that on Lemmy.

You can upload images for your posts (not sure for short videos) and cross-post them to other communities.

Edit: I don't know kbin.social but I suspect it would be more adequate to what you want to do.