I thought the Silvan (such as Legolas) was part of the Avari, but maybe Legolas and Thanduial was Sindarian. I can't really remember. But I think most of the elves of Mirkwood where silvan
Part of, yes. The primary difference being that Silvan elves were under the thrall and influence of the Eldar and so can not be accurate representations of what Tolkien calls "wild elves". Though, in fairness, Tolkien had written that, at least in some occasions, they (Silvan/Avari) cannot be told apart. Which may provide some clue to their guise.
Which is why I mentioned that the Silvan elves are the ones they must have the most in common with. But I don't think they are one in the same. A real world example would be the stark differences between Celtic peoples that had regular contact with the Roman Empire and those that did not.
Legolas and Thranduil (and presumably most of the house of Oropher) were Sindarin/grey elves, and it was not uncommon for wood elves to be ruled by other kindreds (as in Lorien with Galadriel and Celeborn). Thus, Legolas being a princely figure from the start, he also makes a poor example from which to extrapolate what the Avari must have been like.
Still, I'd like to think that they aren't too different from the wood elves. In fact, I'm not even sure the distinction is all that relevant by the time the Fourth Age rolls in (the Eldar having largely faded into irrelevance). Nevertheless, I wonder. Tolkien's world is so complete that any mysteries in it truly inspire a longing for answers, even if those answers can only be derived through speculation.
And there are few things more mysterious in Middle Earth than "the Unwilling".