I've worked with a handful of technologies in my short career. My current position is mostly legacy ASP.NET (framework MVC and a bit of WebForms). Prior to that, I was using PHP and before that I did some really insignificant freelancing using Node.js and a few other JS technologies.
The problem is, it's a pain having such fragmented experience. I don't have a more then a couple years professional (i.e. on the job experience) with any of that tech. The only one I fell comfortable with at the moment, is ASP.NET and C# and hell, I don't even have a PC running Windows at home anymore.
When I stopped paying attention to the trends, I think Node.js and React was the popular stack, angular was (sort of) maturing and Vue was up and coming. I also remember Elixir and Phoenix being touted as a rails killer ans something to watch, but am not sure what became of it. But I'm wanting to learn tech that will get me a job I will enjoy more, but I have no idea what's best to invest in. My jobs have typically been full stack, but I'm more comfortable on the backend. I've looked at learning Python (Flask, Django), Elixir (And Erlang, but when I tried to last, I gave up after having issues mentally parsing the list comprehension syntax) and even Go, but I'm not sure how well the job market is for any of these, especially for someone with no experience. I also have very little experience with frontend JS frameworks, embarrassingly, but those seem to have slowed down form a while back.
What new(ish) tech would you recommend to learn, that will be valuable to employment at least in the short term?
For every single thing that you are able to build, you pick a language or tool that suits you the best and just keep improving. Each language has it's pros and cons but, IMO, what matters the most is the code quality, architecture and the things I mentioned above. Better focus on those skills.
If you write nice structured, good organized PHP code (e.q), that is stable in production and which is easy to maintain then you are far better engineer than someone who is showing off with his Rust or Haskell unstructured, not consistent and in the end unstable code. Don't let the trendsetters put pressure on you.
In the end, if you are are really bored with C#, try to learn some F#. It's also .NET but it's a different type of language and if nothing happens, in the end, you will write better C# :) Personally I think that SQL is also a good investment. Most of the people avoid it, I don't know why, but there are and there will be always jobs for a good SQL developer, especially in big companies where the salaries are also higher.
Good luck :)