Quilcom 4EA+ is a free VSTi synthesizer which used both additive and FM techniques. It comes with 65 presets included. They cover a wide range of types and hopefully many of them can be easily adapted to edit towards what you may have in mind.
There are 2 sound generators, an “Additive” one with FM and a “Single” basic Operator (oscillator with ADSR) also with FM. Both send to the output module which has a parametric equaliser, a chorus/delay and a Reverb in series. The source is monaural, as is the eq, but the chorus and reverb are stereo.
The Additive section has slider level controls for 16 sine wave partials f1 to f16. The Even and Odd harmonics are mostly routed to their own ADSRs. The Odd and Even FM inputs can receive feedback inputs each from either the Odd or Even outputs (pre-fader so the sound is easier to programme).
In FM terms the harmonic series oscillator is a carrier and there are, in addition, 2 modulator oscillators. Since they are modulators you cannot hear them directly but only when they are affecting the harmonic oscillator. These modulators are sent to their own Odd or Even FM inputs but they can be linked. By linking you can use one modulator for both Odd and Even or make use of both modulators in parallel for more complex sounds.
The “Single” operator has 3 FM input options: Self feedback, and from the Odd and Even modulators –provided they are turned on of course. The FM signals from the modulators to the Single are pre-fader so their level outputs won’t affect the Single’s FM inputs.
There is just 1 LFO in the system to keep cpu low. This is on the MIDI panel and can be used to modulate the global FM and/or selected by individual ADSRs to modulate amplitude. Note that LFO-based amplitude modulation of modulators will affect the timbre, not the output amplitude.