Yeah, I agree, the structure of a subtractive synth is fairly straightforward to understand, if you take the time to read up on it (I didn't when I first started ).
Basically, you usually start with an oscillator (might be called VCO for voltage-controlled oscillator), which is a module that gives you a starting waveform, such as saw, square, triangle, sine, or maybe noise; send the output through a filter (might be called VCF for voltage-controlled filter) that attenuates/cuts out some frequencies or changes the width of the frequency band (small width = resonance, high gain = brings the pain; no gain, no pain! ); and optionally modulate/modify with things like a low-frequency oscillator (LFO; inaudible) or an envelope (ADSR; attack, decay, sustain, release).
I think if you get used to imagining what happens through that process, even just how the input sounded and how the output could sound, you can think up sounds and decide "okay, to make this sound, I should probably do this, then this, then this." or "I have this sound somewhere... it's on the tip of my tongue... now where was it?"
FM is a little funky (also an inside joke, 'cause you can make e. pianos with it!), and the theory on that was pretty hard when I tried to read up on it (I ultimately just figured it out by experimentation). It's essentially when a waveform gets stretched and compressed according to the slope of another waveform over time (stretching = positive slope, compressing = negative slope). What you get is a sound that becomes buzzier as the frequency increases; some might call it "gritty" in some cases, or "glassy" in other cases. It gets even more interesting when you change the pitch of either the incoming oscillator or the FM oscillator, since that changes how the wavelengths of the waveform line up a bit, creating less "standard" modulations.
Wavetable is more or less just a way to draw your own waveforms, or use predrawn waveforms. It's not much more than having more interesting "basic" waveforms to begin with in a synth.