Unfortunately it is not only the C++ knowledge, that is in your way, but a basic understanding of audio processing in general.
In one seconds you have normally 44100 or 48000 samples. That means you want to change the slider 48000 times per seconds. Even if you do a highly optimised drawing lets say of 60 frames per seconds, your slider had 800 different values per frame. This shows, that your slider position would end up at random positions. Needless to say, that an interaction with that Slider would be impossible.
Parameters a user can handle should not need an interaction of more than 30 changes per second, maybe even less.
Now the C++ side:
Your parameter needs to be in the Processor. The GUI should only read the value, the processor should not push a value to the GUI, since it is not necessarily present. At least it should not rely on beeing successful doing so.
Also any interaction with the message thread (GUI) should be done with caution, since the message thread does not guarantee to finish in a determined time, which is crucial for the audio thread.
Hope that gets you further, maybe have a thought, what you actually want to control with the slider…