There are a number of large companies sponsoring MariaDB.org development. I don't think there is any concern about its development going forward at this point.
Attempting to get a seasonal job, year after year, for just 2-3 months a year would be difficult.
My wife is a former school teacher and she spent a considerable amount of time planning and working on things for the next school year. If teachers are expected to all work over summer break to make ends meet then their performance will suffer.
I'd argue that a better approach for teachers and education in general would be to move to more year-round schooling.
I have not worked at Facebook but have several acquaintances who have in the past. A frequent complain I've heard is they actually do dogfood their own product. Much of the communication is done via Facebook between employees and is difficult to stay up to date on things.
If you want to get familiar with DevOps basics in a cloud environment I would start with the Cloud Resume Challenge (https://cloudresumechallenge.dev/). It provides a good way to get started on building things with IaC and using pipelines to do so.
It also provides practical, real-world experience that none of AWS' certs offer.