I have 15+ years of experience working on fullstack/frontend. I have never done a really deep backend work. I just built CRUD APIs, using message queue, cron jobs, etc.
I'm interested in going into the backend to learn more. However, given my experience, it's really hard to find a job even one with a lower level.
Is it too late for me to switch? Has anyone done this before?
I've made similar pivots in my career as a full-stack dev. I went from a mostly web-based FSE to a fully mobile FSE. I talked up my mobile experience in my resume and applied for roles that were mobile-heavy, but listed as full stack.
In my opinion, full-stack hires are often "unicorn" hires, so it's easier to land a role without having deep experience in one area of their stack as long as you show competency and experience in other areas. If I were in your shoes, I would apply for backend-heavy full-stack roles for your next job, and then jump from that role to a pure backend role.
A late-stage career pivot is tough but definitely possible. Given how senior you are, you should have a bunch of general technical fundamentals that make it easier to pick up any new language/tech stack. Interestingly enough, a recent thread talks about this: "How much does language-specific experience impact switching to other tech stacks?"
What I will say is that you should do this pivot through an internal team switch, not an external interview jump. When you join a new company entirely, they are already taking a risk by not knowing if you'll be able to adapt to their culture. Very few hiring managers want to stack the extra risk of not knowing if they'll adapt to the new tech stack as well. More thoughts here: "How to transition from back-end development to distributed systems?"
Unfortunately, internal transfer was equally tough as teams are looking for system engineer not a product engineer.
I believe I can pick things up quickly but it’s really hard to get an opportunity to do so.
It's rough as the current economy isn't one where companies are willing to take risks on employees 🙁
Sometimes you need to play the long con and switch to another (often bigger) company with your current tech stack and then do a switch there after establishing yourself as a high-performer across 1-2 years. Big Tech companies like Amazon, Google, and Meta are known for having good internal mobility (it's much weaker now but still industry-leading in relative terms).
One way to satisfy that learning itch immediately though is by building stuff outside of work (if you can find the time). The 2 main paths here are open-source and side projects: