The DX-21 is basically a blown-up version of the DX-100, with full sized keys, and additional storage. The editing and programming is identical to the 4 operator FM synthesis in the DX-100.
Gabriel purchased the DX-21 and went deep into the same sound designing rabbit hole as I did back in the late 80s. We both had a similar attitude in that we tried to make sounds on it that it wasn’t really designed for. I purchased my DX-100 shortly after playing a Roland D-50, and found the DX-100 lacking. In Gabriel’s case, he probably would have probably done better with a few nice analog synths.