If you want “true to life” Yamaha FM, then you all should just buy a TX7.
You can only upload patches with SysEx
BUT
and its a BIG BUT
that space on top of the TX7 is the exact size of an Elektron box.
It fits perfectly flush.
It is the most space saving and useful implementation of DX7 style FM you can get, if you have an Elektron box.
TX7s can be bought for $80
You can buy a backlit LCD screen for $20
cheaper than the Volca, authentic DX7 mk 1 tone, easy to find space for.
Loading patches is moot, nothing about FM is “tweakable”, so just upload some new age conga tones and sequence into oblivion
EDIT: The DX7 mk1/TX7 also process the end stage of each voice in the polyphony separately in analog, then sum together. There isn’t MUCH more processing than just a simple out, but it does make an effect on chords. You can hear separation, which is hard to achieve even in something pricey like Omnisphere. You can also live tweak in an editor, its not JUST SysEx patch dumps, but no on-board editing.