I seem to be stuck in a sort of plateau when it comes to gaining experience in Unity. The problem is, I've been making games without really learning anything new and really want to get out of the making-bland-simplistic-games loop. And yet, all simple Unity tutorials are too easy for me, and all more-complicated tutorials(e.g. the ones by Jason Weimann on YouTube) have content that is either too difficult or irrelevant for my development.

Does anybody know any good free tutorials for intermediate-level Unity?

If it helps to be more specific, I've been particularly struggling with GUI. 

I’d suggest to start relying less on tutorials. You’re not going to find one for every small detail and feature, and relying on them will prevent you from truly getting to make what you have in your imagination. Programming is, after all, just problem solving (computers are just a bonus). You don’t always get to have others solve problems for you.

Similar to Dark Dimension's suggestion, what I would do to learn new Unity features is take an existing game and try enhancing it with the new feature, e.g. today I'll try adding dynamic lighting (shadows), and then I'd read the Unity doc (Unity generally has excellent documentation, it amazes me when people don't read it), and any other useful articles resources like their blog articles or forum discussions, then I try it out, and that usually results in going back to the aforementioned resources to figure out problems and understand optimizations, and then once I had the tech under control there's more widerspread research involved in how to use it properly (lighting is a big topic!)

That doesn't really answer your question on tutorials, but I don't like video tutorials (and most Unity tutorials seem to be video) and I don't like Unity GUI. I think they made a mess of it, problems including it's a lot of work just to do something simple, and you can't just code it, you have to do all this manual drag-and-dropping of assets (so I can see why you might want a video tutorial!) and thus it's hard to make anything reusable. I suppose if you want a good-looking GUI you have to use it, but last I checked (which was at least a year ago) yet another GUI system was in the works, and if I wanted to just concentrate on the rest of the game I might try to get by with their original all-scriptable GUI API which I think can still be used in-game

https://docs.unity3d.com/Manual/GUIScriptingGuide.html

and in fact I used it for a sample pause/options menu in my now-obsolete Unity book, but I converted it to C# a while ago, might be usable

https://github.com/technicat/learnunity/blob/master/chapter17/unityproject/Asset...

I found switching  software taught me the most.  Every software teaches you something different and even if you return to the software you were using - it gives you a better understanding of what is available and new things to discover.    It refreshes your thinking  because you could learn something new in each different game development software.  

There's free software like Godot and then a lot of paid software have free versions or trial versions with some examples and tutorials that teach you the software, for example, GameMaker and Clickteam Fusion, so you don't have to spend money.

However, on the Unity side - I completed the Junior Programmer pathway from Unity Learn- it gave me a good insight into what the software has to offer and even though a lot of it was not what I was interested in,  it helped me to learn the basics of programming in  Unity and  gave me some fresh ideas  Junior Programmer - Unity Learn

 I also enjoyed this one: Real-time Animated Storytelling - Unity Learn - it gave me insight into a section of Unity I wouldn't have discovered on my own. It helped me to think of my games in a different way - many of my games have a story element - but this would allow me to animate the story, not just show the text on screen - it's something extra that I can do.