if you want to build a DAW, use the tracktion engine. It will take care of pretty much all the audio side of things (though its still good to know whats going on behind the scenes) and you will really just need to worry about the UI and connecting the UI to the engine. So to get started you could focus on learning JUCE and how to build a UI in juce, then move to integrating with the engine.
The engine has some tutorials and a lot of helpful examples. It doesnt really have documentation though which can be a challenge at times if you dont really know where to look to find something, but the examples help quite a bit.
As far as learning JUCE, the juce tutorials are probably what helped me the most. I did the free kadenze course as well to learn a bit about plugins but as you said its not really that great. Definitely wouldnt pay for the paid course. The audio programmer youtube videos are good if you like to watch video tutorials. However yeah they arent really that in depth and a lot of them are essentially just videos of the written juce tutorials. I personally prefer the written juce tutorials since the videos can be sort of slow moving.
I was honestly kind of shocked how hard it is to find decent audio programming tutorials and books and what not. I was used to more traditional programming topics where theres a plethora of free info online for almost everything youd wanna know. Definitely not the case with audio. Audio programming is super niche so theres just not much out there.