There’s another thing you can do in addition to checking Disable Waveform Interpolation to have a sound like the old FM synths.
Select the Bitcrush effect, apply it with 100% wet 0% dry and the following settings:
Frequency: 0
Bit Reduction: try different values from 35 (subtle) to 127 (exaggerated)
Cutoff: 127 (disabled)
That will simulate quantization noise on the envelopes and DAC, creating the famous “zipper noise” you hear on most YM2612/OPL2/OPL3 sounds.
Also sound design plays an important role, limit yourself to 4 operators and a single voice per patch I’m sure you’ll get enough grittiness for your liking 