What is wrong with my custom token to embed entities via token?
See the example : https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Utility%21token.api.php/function/hook_token_info/8.3.x

Inside tokens there must be 3  levels, tokens][token_type][token].

Also tokens don't usually work like that and it will not be possible to validate that token, it is always a token type and that has tokens, which can have a type which then again has tokens. So you'd need to define types for your nested structure.

Not quite sure why you aren't using the entity_embed project to inject tokens, I assume this is supposed to be used inside a formatted text. https://www.drupal.org/project/entity_embed