Bullshit... quit buying into that crappy old meme. At best, that might be sorta true IN Silicon Valley Proper, for a subset of companies. In the rest of the Real World, there's nothing particularly special about being 30 in tech.
If you're in SV and you buy that, then my advice would be move somewhere else. If you're not in SV, then quit worrying about being 30.
I'm feeling overwhelmed by the sheer number of technologies out there that I "should" learn: Python and R (and their libraries), Spark, Hadoop, TensorFlow, and pytorch. You also need to know how to productionalize things: Docker, AWS, distributed databases, the latest web stuff (sockets?), etc.
You don't have to know all of that stuff. A given firm, for example, probably uses Python OR R, but probably not both. Learn one or the other, you'll be fine. And not all roles require you to do everything from writing Dockerfiles to writing Map/Reduce code in Hadoop, to working with Tensorflow or PyTorch. Which bits of the stack you need really depend on the specifics of the job you wind up going for. But don't put that much of a burden on yourself to think that if you don't know every single thing on some "list" that you aren't employable.
I mean, you can never be sure, but I can just say that, being well into my 40's myself, and working on the East Coast (RTP, NC area), and having worked for everything from tiny startups to giant multi-nationals (in tech and out of tech), I haven't seen the kind of rampant ageism you hear described on here. And I have friends who work at Microsoft, Google, etc., and I haven't heard anything from them to suggest that it's rampant either.
I get the sense people won't take me seriously if I'm starting in a junior role at 30+ years old
People change careers all the time, for various reasons. And yeah, maybe some people will look askance at you for it, but not all will. Now, if you're betting the farm on working for ONE specific company (let's say Google) than the reality is it might not happen. But it might not happen for lots of reasons. But if you take a broader view of things and don't hang your self-worth up on working for a "FAANG", you should be fine. Work for an insurance company, or an auto parts retailer, whatever. The world is a lot bigger than Apple, Google, and Facebook.
Nobody is saying that "age discrimination in tech is bullshit." What is being said, and what I believe is supported by the weight of the evidence, is that in most of industry (eg, outside of Silicon Valley), it is not the case that "30 is old in tech."
The "tech scene" is a lot more than just SV, and the kinds of things you hear about as common-place in SV just don't seem to be so common elsewhere.