[Legate of Amon Lanc]

Indeed. But he was not going to change into Ringwraith unless maybe if he had the Ring for another five hundred years or something like that. Gollum had the Ring for 478 years, if I count correctly, and even if Sauron gave the Rings to the Nazgul right after he acquired them* (1697 Second Age), the first appearance of the Nazgul as wraiths is dated to 2251 - so this is 554 years for the Nazgul to wear them, not much longer than Gollum did. And he certainly seemed still quite "okay" (in the terms).
With using the Bilbo and Frodo example, I wanted to lighten the things a little bit, of course they didn't have the Ring as long as Gollum. But Frodo was, as Gandalf said, "on the threshold of the Shadow-world" when coming to Rivendell, whilst Gollum was still perfect living and breathing creature as much as he could.
And, the "green slimy creature" - first, I don't think it is anywhere written that Gollum was green. Second, I think his change was mainly given by the environment he lived in, he might have looked like death, but try living in a cave on the edge of a slimy lake for five hundred years eating only fishes and some orcs from time to time, and you will look no better than him. So I think this is not the answer.

*which I doubt, by the way, I think he had his mind on the war at that time. I find it likely that he gave them to the Nazgul sometime around 1800 when he "started to spread his influence to the East" and also shadow fell over Númenor - and we know that three of the Ringwraith were three "great Númenorean lords", and he couldn't reach them until they started building their provincional cities in ME - which is exactly what they did around 1800 SA. In this counting, the Nazgul would have worn their Rings for only about 450 years till their transformation, which is less than Gollum did, and he still had not "faded" (as Gandalf said).