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: For many years, Neo-Sindarin writers used 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 was likely floating around for much longer. For “plume”, I would use an augmentative form: ᴺS. pesson “plume, (great) feather”. In 2024, a native Sindarin word S. laud “feather” was published in PE23; see that entry for discussion. Pess “feather” has been a part of Neo-Sindarin for so long that I think it is reasonable to keep using it, but if you want to stick to Tolkien’s own words, laud is probably better.
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).