Get base64 photo from iOS photo library uri

I'm building an app in React Native, and I need to get an image from the Photo Library as base64 using the photo uri (e.g. photos://A2B9B28B-9A3E-4190-BCA0-B11F1E587085/L0/002). I was originally using the Asset Library but I've updated my app to use the new Photo Library and I'm struggling to get it to work.

My old Asset Library method was:

{
  NSURL *url = [[NSURL alloc] initWithString:input];
  ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
  [library assetForURL:url resultBlock:^(ALAsset *asset) {
    ALAssetRepresentation *rep = [asset defaultRepresentation];
    CGImageRef imageRef = [rep fullScreenImage];
    NSData *imageData = UIImagePNGRepresentation([UIImage imageWithCGImage:imageRef]);
    NSString *base64Encoded = [imageData base64EncodedStringWithOptions:0];
    callback(@[[NSNull null], base64Encoded]);
  } failureBlock:^(NSError *error) {
    NSLog(@"that didn't work %@", error);
    callback(@[error]);
  }];
}