This root first appeared as ᴹ√SED “rest” in The Etymologies of the 1930s with derivatives like ᴹQ. sére “rest, repose, peace” and N. sîdh “peace”, along with the name of the Valië ᴹQ. Este < ᴹ✶ezdē (Ety/SED). The root √SED “rest” along with the etymology of Este reappeared in both the Outline of Phonetic Development (OP1) from the 1930s (PE19/45) and the Outline of Phonology (OP2) from the early 1950s (PE19/92), and again in revisions to the OP2 page annotated in green-ink and thus probably from 1970 (PE19/91 and note #110).
The root √SED “rest” was mentioned a couple times in the Quenya Verbal System of the 1940s as the basis for the verb ᴹQ. ser- “rest, repose” (PE22/102, 125) and again (unglossed) in the Quendi and Eldar essay of 1959-60 as the basis for Este (WJ/403). In the sense “peace” this root had some competition, however. In the first version of the Gloria in Excelsis Deo prayer from the mid-1960s, Tolkien used Q. sívë for “peace”, revised to Q. sérë in the second version and then Q. rainë (VT44/32). The word Q. sívë “peace” reappeared in unpublished etymological notes from 1969 derived from the root √SIB “rest, quiet” (VT44/35).
Neo-Eldarin: For purpose of Neo-Eldarin, I would use √SED for the act of resting and so Q. sérë “peace” = “restfulness (personal peace)”, and would use √SIB = “quiet, stillness, absence of activity” and so Q. sívë “peace” = “absence of activity or violence (environmental peace)”. I would use Q. rainë as “good will” in connection to √RAY* “smile”.