Vim plugins are available for most IDEs. I almost never use vim (except for the shell), but I use vim-style editing in all the IDEs I use.

Is that a viable way to learn vim? As an advanced jetbrains poweruser

Absolutely - I first learned "vim" using a visual studio plugin. Since then I used it in 5 other editors, but I barely used vim itself.

The main downside is that you don't learn things like window managing, file exploration, etc - mostly because other editors are already good at that, and it's not something plugins focus on.

Is it worth it? I am already pretty fluent using my custom-ish shortcuts for all navigation, will vim speed me up at this point?

That's how I learned it. I got the extension for VS Code and just started playing with it. If I got frustrated, I would slap "i" and go back to my normal way of doing things. The frustrations went down and now I use vim bindings all the time. I'd also recommend getting a Vim plugin for the browser (https://addons.mozilla.org/en-US/firefox/addon/vimium-ff/). Totally optional but it really helped click in the j and k movements.

Doing it that way will at least prevent you from being disappointed due to missing features. Vim plugins are nice and I definitely prefer them over basic input, but almost all of them don't even cover the basics.

I've learned vim in the standard vim editor at the time, so I can only guess but it should probably work. 99% of my work with vim is navigating and editing text. The rest is macros and substitute, all of which is available in IntelliJ. The things you miss out on are (I think) colorschemes and vim specific plugins (which you mostly don't need because you have a fully fledged IDE).