Whats hard (and impressive in regards to those who came up with meaningful patches in the 80s) about starting with FM synthesis isn’t only the programmability (modern FM synths have that covered, see the other comments). It’s not an intuitive thing to work with algorithms and you need to learn what is happening.
I can recommend FM Theory and Applications by Musicians for Musicians for everyone who wants to create own patches:
In addition to this wonderful resource, I can also recommend getting Dexed and just playing around. It's free, open source and, importantly, as unintuitive as an actual DX7. Great for digging deep into how and why a change in an operator creates a change in a sound.
As I noted in another comment, you can start out, and get pretty far, with just a single operator pair, which basically determines the waveform and spectral complexity. For me it has a similar feel (unsurprisingly enough) to analog FM, ring modulation, or oscillator sync.
I can recommend FM Theory and Applications by Musicians for Musicians for everyone who wants to create own patches:
https://www.burnkit2600.com/manuals/fm_theory_and_applicatio...