Ask HN: Best long-term and marketable web stack for freelance contract devs?

I feel like I don't get enough time to deep-dive into one specific programming language / framework, since as a freelance I feel I need to learn 4 or 6 or 10 languages / frameworks and need to keep up-to-date on all of them.

I want to invest the next years learning a few carefully selected languages / frameworks in more depth, without worrying too much about breaking changes in the underlying software, so that (optionally) I can easily pick up other languages / frameworks on-demand, since I understand the underlying concepts better.

Do you have any go-to web dev languages / tools / frameworks that you feel will be a good investment of your time in the long-term?

Here's some stuff I like: Python / Flask, PostgreSQL, AWS (incl. ElasticBeanstalk) / Netlify, Mithril.js, npm / Makefile / Parcel.js. (Currently I use a lot of PHP / Laravel / Symfony, Vue / Angular in addition to the previously mentioned.) Still undecided on Docker, but I do use it quite regularly, but also still use Vagrant as well.

(Re: My background: Freelance web developer / software architect in Western Europe, mostly working with agencies and SMEs; I've been doing this for about 10 years, 6 of them full-time, but have been developing websites or doing IT admin work on the side for about 20 years.)