I'd add one more quote to what you've already mentioned, Pete; or perhaps a better phrase might be to expand upon one of the quotes you've already mentioned:
'Elric stared backward at their pursuers...he had a vague impression of the riders who raced behind the pack. They were swathed in dark cloaks and carried long spears. Their faces were invisible, swathed in the shadows of the hoods which covered their heads...zombie-men, their dead eyes eerily luminous...'
This doesn't give us a lot to go on, but at least it tells us that the 'Lords of Dharzi' (which is presumably a place name, rather than the name of their race, as Moorcock doesn't call them the 'Lords of
the Dharzi') have at least the semblance of human form.
But as you and Loz say, that's really about all we know about them.
In his scenario 'Stolen Moments' in Chaosium's
Stormbringer supplement
Perils of the Young Kingdoms (1991), Australian writer Nick Haggar describes the Dharzi as having 'an alien cast to their features...broader in the chest and slightly smaller' than an average human. Another of the Dharzi, the ancient sorcerer Pemmnetr, appears in Ken Rolston's even older
Stormbringer supplement,
Black Sword (1985) but his description in that book is so cursory as to be almost non-existent.
I don't know whether it was Ken St Andre or Steve Perrin who was responsible for linking the Dharzi to Melniboné's slow decline in his first edition of
Stormbringer (1981), but certainly one of them did; because here's what that original entry says:
'A millenium before Elric's time, the Silent Lands were the great homelands of the Lords of the Dharzi, a non-human race who worshipped and partially controlled the Beast Lords. The Dharzi had outposts throughout the world, but when they began to threaten the primacy of the Bright Empire of Melniboné, they started a great war between the two Empires. Eventually the Dharzi were defeated by the Melnibonéans and almost wiped out.'
So again, no dates or details about the duration of the Dharzi war from this secondary source, though it's worth noting that St Andre/Perrin erroneously name the Silent Land as the 'homelands of the Lords of the Dharzi' despite Moorcock being uncharacteristically clear in saying, as you've already noted, that there 'was never contact between the Dharzi and the dwellers of this [Silent] Land'.
I agree with Loz that 2000 years seems too long for the war; 100-200 years at most would be more realistic, I think.
And the suggestion of seeking inspiration from the three Punic Wars between Rome and Carthage is a great one, Loz; it suggests a series of wars over an extended period; perhaps on a number of fronts (even potentially on other planes) before the conflict was finally resolved...