Site Availability and Broken Links?

Availability is a huge topic. If you are looking to have high availability I would recommend running multiple canary tests per minute. Also when you deploy your app, I would recommend doing first to one box, gather metrics, and only if everything is fine doing a full fleet deployment. The good thing is you can automate these things.
Also is nice to have alarms in your metrics that page you or someone that can fix any issue right away. You can have alarms on API failures, log level error messages, low traffic, client side errors, etc.
Just in case you miss something with your tests, you can setup google search console, which tells you every time they find a broken page.
Best 👋

Uptime Robot helps check for basic availability. Haven't thought about the other things.