Hey, I'm a long-time music producer and I'm dying to get into this exact kind of work - writing plugins and eventually synthesizers from scratch. I'm curious what your path was to get to the point where you can write an audio plug-in from scratch - I noticed you're using the JUCE framework, and that's about as far as I got and I never really escaped tutorial hell.
When starting out, the first plugin I made was a gain/panning plugin, then a simple saturation plugin. These are good ones to start out with, since the output for a sample only depends on the input of that sample, and not the samples before it. After that, I would recommend making a delay plugin: there are a lot of opportunities for creativity with delay, once you have the basic code down.
The plugin project structure can be a bit confusing at first, especially in the interaction between the GUI code and the audio processing code. The tutorials are helpful for that: once you've copied a tutorial, you can try expanding it, adding more knobs etc.