The free software to edit DX7 patches is called “Dexed”: Invalid Link Removed
Honestly tweaking a DX7/FM patch is super complicated, not at all as predictable as a typical analog/subtractive synth engine is, which is why most of the songs that used the DX7 in the 80s/90s were using straight preset sounds (the reason the synth bass, electric piano, and lead key lines on so many songs sounded identical in the 80s). Part of that was that editing on the DX7 itself was something of a nightmare, but also because it took a deep understanding of the way it works to get anywhere useful with it, and it is not nearly as intuitive as subtractive synthesis IMO. If you search Google for “DX7 SysEx” or “DX7 patches”, etc you will find lots of sysex patches that apparently should be uploadable to the FI with the new software. If there are 99 FM patches already loaded in the new firmware then I would assume most of the classic DX7 bass sounds are already accounted for. I’m looking forward to seeing what else they cooked up- FM sounds can get super crazy!