Any recommended books/courses for someone in my position? I am trying to avoid the beginners 101 and learn javascrip intros (looking for something more advanced).
You can also pay someone like Tailwind for a license to their UI kits which grants you access to premade components and full website templates.
As a backend developer; picking up typescript shouldn’t be too difficult for you if you leverage reference points like V0.dev or ShadCN.
I learned all my frontend stuff on LinkedIn Learning (formerly Lynda). My colleague likes frontendmasters.com. MDN is a good resource: https://developer.mozilla.org/en-US/docs/Learn/JavaScript
If you're used to writing a lot of APIs and backend stuff, the frontend world is different in that it's a lot declarative markup and not as much logic. There's almost no OOP in frontend anymore; it's mostly just a bunch of HTML (or JSX, which is basically React-in-XML-that-looks-like-HTML). The JS array and object "iterable" methods (map, filter, flatMap, etc.) are used a lot, but not necessarily for data manipulation, just to put API response objects on-screen (e.g. you map through an array to transform each data object into a UI component).
Take a look at some of the example MUI templates to get an idea of how these things are put together. Once you do one or two it should become pretty straightforward.
There's a lot of 80/20 stuff in the frontend, as in you can get the basics of it pretty quickly, and then spend a lifetime diving into CSS rules and all that. But with MUI, you don't really need to drop down to that level of detail – it's all abstracted into well-documented and well-tested components with their own APIs. Modern React is pretty straightforward compared to how it used to be. Or you can check out Vue/Svelte, but they have much less mindshare and ecosystem support than React. There's also Angular (also unpopular now) and I just heard of https://vaadin.com/ for Java.
Edit: Keep in mind, though, that if you want to use React, I'd consider it a different learning track altogether vs barebones JS. In React you almost never directly mutate the DOM (document object model, the parsed node tree of HTML) the way you do with vanilla JS.