frankly I highly doubt that the reason why China is not mass producing WS-20 is that they thought "hey WS-18 is good enough for our needs", no the WS-18 is not good enough for China, if it was then why even bother developing the WS-20?
it's always like this with Chinese & Russian netizens, whenever they talk about a weapon that they still can't mass produce or haven't finished developing, they say "we don't have it not because we can't produce it yet, but because we don't need it", but then once they produce it they say "oh we actually do need it".
take for example the aircraft carrier, before China started building the type 002 the Chinese used to say "naaah we don't need aircraft carriers that is why we are not building them" but then once the Chinese began building them, they made fun of the Russians when the Russians said "aircraft carriers are not needed".
now to this day I still see some Chinese netizens saying "hey we don't need to mass produce nuclear submarines, conventional submarines are enough", they don't want to admit they can't mass produce them (the type 095) yet, but I assure you, once China does start mass producing the type 095, those commentators will begin saying "having SSNs is of utmost importance".
same thing goes for the WS-20, you say the reason why China isn't mass producing it yet is not because it's not ready, but because they don't need it since WS-18 powers both the H-6k & Y-20 to save money on logistics (as if China is some dirt poor nation that can't afford to spend money on logestics for an engine, and not the second biggest economy), but I assure you, once China does mass produce the WS-20 while the H-6 would still be in service, you will change your statement.