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.