Oh I love FM synths! I'm working on a customisable one in my spare time lately for the kids' school, as the music teacher was complaining that the students have been using all the same samples over and over. Feel free to have a peek! Desktop only. Source code is hopefully nice and clean too: https://chrishulbert.github.io/you-synth

Ah Operator. This synth is so deep. Not only is it a fantastic FM synth, but it does subtractive synthesis well too. Also, it really is impressive how the UI manages to fit all those parameters. I mostly use it for cool synth leads. Here's one of my favorite videos on Operator https://youtu.be/rfeY0_k1ctk?si=s68Lr033cHf34a4M by Robert Henke himself.

I love FM synthesis and I love Robert Henke. Great article