alt Hacker NewsIf you want the most job opportunities I would bet on the web and database backend. Almost all mid to upper size businesses has some custom made web solutions which stores to an SQL Database. There are lots of sub-areas within this domain like cloud, security, different databases, scalability, authenticatin, front end and the list goes on
I'd say focus on databases, especially as someone nearing middle age.
Front end/web work is a treadmill. Web frameworks come and go. SQL and relational databases have been a foundation for applications since the 1980 and that is still the case today. The investment you make in learning something like Postgres won't be something you have to throw away in 5 years when the next hype cycle around a new framework comes around. Basically after 30+ years in software, the one thing that is still relevant and that I can still do well today is work with relational databases. I got tired of running the web framework treadmill and am now years behind whatever people are using these days.
Edit: also linux/unix shell utilities. Awk, sed, grep, xargs, and others form a surprisingly capable toolbox for filtering and shaping data, and they all date back many decades. I've never had to throw away my experience using these tools.