Pushing Content from SharePoint 2013/2016 to Mobile App

Hi

Need to check whether a Solution we are proposing is the best method. 
I am not a developer so might need some guidance on the best way to suggest a solution. 

We have an OnPrem SharePoint 2013 and 2016 farms.

External users need access and cant be added to the client domain and form's auth is not an option. 

There will be content that gets published and approved and that need to be somehow published to the mobile app.

A developer has proposed a mobile app that can pull data via REST/SOAP. or what API?
Firstly looks like SOAP has been depreciated?
So will rest be the best option? 

Then we are thinking of Developing a TimerJob that can just push the data to a SQL DB and then the App to pull it from the DB?

Before we reinvent the wheel, or maybe there are easier ways to accomplish this I would love some feedback.
Would also be good if we do this in a way that will work if they migrate to SharePoint Online. 

Thanks in advance.