I can dig that - I actually did enjoy the sound of the DX-100, it just had that ‘older’ sound to it.
I had thought about your reply last night, and came up with one other idea for generating FM tones, actually, maybe two/three.
In the single-cycle waveform territory, there’s Hardcore / Softcore. Not fantastic, but fun:
https://www.floats.se/software
Much of these could probably be generated using the internal FM waveform generators we have for Renoise, like Custom Wave Synth or MilkyWaves. I used MilkyWaves all the time, because the ‘noise’ generators create all sorts of FM tones, instead of actual ‘noise’. Beautiful sounds.
I think I’m probably getting too far off of the path here, but VCV Rack (free modular synth) also has several ways of creating tones in just about any form of synthesis you’d ever want, and all for free (mostly, unless you really want to buy some module).
I’m sure you probably know of all of these, but there’s so many ways to do this. If you’re really gonzo (like me) about waveform generation, and need a particular sound, then you may need to work some filter magic on particular sounds to give them an ‘old’ tone. Filter being generic for ‘any kind of audio processing’ that gives a certain timbre to a sound. Personally, I’m usually striving for that OPL3 sound from my old Windows 95 machine, from some off-brand sound-card in my family’s first computer. Sounded like Autechre/Aphex Twin to me way back then. Sure, I can use other people’s samples from the soundcard, but where’s the fun in that? I want to make 'em myself 
AND!!!
One more, Beepbox. You can do additive and FM synthesis with this, make a sequence and export the sound to .WAV. Free.