I am curious why you're transitioning off from Datadog?
A monitoring solution based on Grafana and Prometheus tends to have a high initial setup cost plus on-going maintenance since it's self-hosted and lacks some of the features that Datadog offers out of the box (alert customization for instance).
I have both worked with and interviewed people in their 40s when I worked at a FAANG. Despite what's advertised by HR, culture tends to be org/team-dependent at bigger tech companies. I believe that you should be more concerned about culture fit which is something you can assess during the interview. If you decide to interview for an individual contributor position, I recommend that you brush up on algorithms. You can either target a company on LeetCode or go through a generic list of problems (here is the one I use https://theinterviewlist.com)
In addition to the resources others have suggested here I recommend going through this list of problems that I put together for the coding portion of FAANG interviews: https://theinterviewlist.com