Me : What the BEST way to create content? 

Myself : Your organisation structure 

Me : ??? 

Myself : The most important messages are in-person.

If you can create a 1-1 mapping of senior and junior employees in your organisation, nothing like it.  That will work wonders for you without you creating a single training content.

Think of a matrix organisation structure with project managers and people managers where project manager will only think of the project’s success (which is project centric and can be short term or long term) but people manager will think of people as human capital. They don’t worry about the project but think of people and their development. A people manager role is in short a kind of a senior mentor to the employee. If you add this role in your organisation, this will be the most effective training process.