I have a DX7 on my shelf over there that I haven’t used in years, but it was great for all these years since I bought it. The DX21 is just a shadow of the DX7, just so you know. Turns out I have FM8 and never used it, so I can’t compare!
The thing about FM synthesis is that it’s not at all intuitive… tiny tweaks make a giant difference. If you’re striving for natural sounding real instruments, then sampling has superseded FM.