Okay! I have some great news!
I’ve removed the OSVR and SteamVR plugins from the project (they aren’t needed). I also unticked the packaging options for them on the android settings and the app now DOESN’T crash on startup!
I’m left with some UMG stuff to sort out (but that’s something else haha) but the app works!
Thank you so much FilAl, you’ve been VERY helpful and very patient. I can’t thank you enough for helping me through this! 
You’re a legend 
I found a solution here: Android app failed with error: library libGLESv3.so not found - Mobile - Unreal Engine Forums