This setup doesn't provide high level of security or any high availability. After actually making your product these are probably the two most important factor

This setup doesn't provide high level of security or any high availability. After actually making your product these are probably the two most important factors for most SaaS startups. If you screw up availability you can't get customers - your site will fall over every time you push any marketing. If you screw up security you can't keep customers - you'll fail any kind of due diligence or you'll burn your reputation. Using third party services that do the hard parts for you is very, very sensible. There's nothing wrong with a boilerplate starter kit if you understand the hard parts and just want to go faster. There's nothing wrong with using starter kit for a prototype. But if you actually want to build your app, and you think you need this, you should lean towards using something that manages this sort of thing for you instead.