The word G. pectha “plume” appeared in the Gnomish Lexicon of the 1910s, along with G. pecthon “plume, feather” (GL/63-64), clearly cognates of ᴱQ. pekte “plume” under the early root ᴱ√PEKE (QL/73).
Neo-Sindarin: Somewhat surprisingly, there are no later “plume” or “feather” words in the currently published materials from Tolkien’s writings on Noldorin and Sindarin of the 1920s or later. Most Neo-Sindarin writers use ᴺS. pess for “feather” as the analog of word Q. quessë, which Tolkien used in the 1930s-1960s. Hialmr added this Neo-Sindarin word to the VQP (VQP) in 2015, but it has likely been floating around for much longer. As for “plume”, I personally would use an augmentative form: ᴺS. pesson “plume, (great) feather”.
A noun appearing as cwathli “plume, spray, tassel” in the Gnomish Lexicon of the 1910s, related to G. cwas- “wag, shake, flap” (GL/28) and thus almost certainly derived from the early root ᴱ√QASA which likewise was the basis for “shake” and “plume” words (QL/76). It appeared as cwathil “a plume” on a different page of the Gnomish Lexicon (GL/28).