Disse isn't probably a good word to go by. I'm fairly certain it's an adverbial form of
dis, as the meanings are very close (replace "only" or "just" on the example sentences with "simply" and you'll notice the meaning remains almost intact) and geminations tend to happen and disappear and geminate and /-e/ seems to be a common pattern for deriving adverbs. You'd more likely end up saying "ordinary" than "unique". Note, though, that "simple" does not necessarily carry as much dismissive weight as it does in English. Dothraki like to have things plain, simple and straightforward. Even their word for "comfort" is
athdisizar.
Can I simply say disseat?
(Simply ... hehh) As per above, deriving forward from an adverb form is iffy at best, but if you could, you'd need to use /-lat/ as
disse ends in a vowel.
I'm wondering, though, is "legendary" really that close to "unique". To me "legendary" is pretty much an accented version of "famous", and we have a word for that,
hakeso. I'd go to "song-like" or "song-born" or something like that before I'd venture to "unique".