Self taught backend dev here. What's the best DIY way to get a CS background?

I'm also self taught (to an extent).

When I was making the transition to tech, I dived into the fundamentals very deeply with a collection of MOOCs (CS50 by Harvard is a great place to start regardless of your current skill level).

I also explored several MIT open courseware materials. There's a great experiment by this guy that's basically answering your questions directly: https://www.scotthyoung.com/blog/myprojects/mit-challenge-2/

If you're not familiar with object-oriented (OO) fundamentals, I'd probably start there since you're on the backend and will probably be exposed to OO eventually (perhaps already).

George Varghese's Network Algorithmics is a good book for backend-oriented people to learn various fundamental CS algorithms.

It presents various CS topics from the perspective of backend optimization, so it's a good book for approaching theoretical CS from a background you already probably understand.

But, ultimately, the best asset for somebody with a CS background is not so much having immediate knowledge, but knowing where to acquire knowledge as necessary. If you have a general idea that for a specific scenario, you can acquire X knowledge in Y resource as you go along, then you're already doing quite well.

Is there any opportunities to get a mentor in your current workplace? Most of what you'll be able to learn is from books and the existing codebase but not all of it, there will be places where decisions were made that cut corners and for good reason - but only people who have worked on it will be able to tell you why and what you should take from it.

I would actually love to know what resources you used to teach yourself backend development if that is possible!

To help out with your question this https://teachyourselfcs.com/ is a resource i have heard good things about.

Find an online course/material on data structures and algorithms.

Learn a lower level language (like c or rust)

Get up to speed on design patterns and OOP theory.

Hey, Quick question how did you get hired while being a self taught developer

I'd check out Coursera, or get my hands on a good book.