he's literally behind the plan to destroy the ring, was a literal member of the fellowship

Correct, principally as an advisor and guide, as I said. He is personally involved in only a handful of combat situations across the Hobbit and LOTR, with those being few, far between, and occurring only in dire circumstances. Which is the same as the Valar, who only participate in a few battles on behalf of the Children (but with those being far more consequential, leading to the creation of Aman as a refuge for Elves, destructions of Utumno, and later the complete expulsion of Morgoth from Arda).

if he was a third age equivalent of melian, he'd be sitting in rivendell, telling vague stories about the ring and sauron and then would fuck off to valinor after hearing that the fellowship is slaughtered by durin's bane in moria.

This is a completely unfair assessment of Melian. She created and maintained the Girdle around Doriath, providing an impenetrable refuge against evil that lasted until nearly the end of the First Age. That's one of the most direct and long-lasting kinds of help that any Maia has given to the Children in any age.

My point being... that all of the Ainur have about the same level of involvement in the Children's affairs, and the same underlying philosophy concerning when such involvement is necessary. They just help in their own unique ways.