Tolkien used √SI as the basis for “near demonstratives” like “here” and “now” from very early in his writings on Elvish. The Qenya Lexicon of the 1910s had two competing roots ᴱ√HYA “this by us” with derivatives like ᴱQ. hyá “here by us” (QL/41) and ᴱ√KI “this by me” with derivative ᴱQ. tyá (< ᴱ✶kı̯-ā) “now” (QL/41, 49). Indications of the latter can be seen words in the contemporaneous Gnomish Lexicon such as G. cî {“now” >>} “here” and G. cîrin “present (place or time), modern” [gloss deleted] (GL/26). However, Tolkien also introduced a new root ᴱ√si(n) “this here by me” with derivatives like G. sî “here” and G. sith “hither” (GL/68). Revisions of Gnomish ci- word glosses indicate Tolkien was vacillating on which forms were temporal and which were spatial.
In The Etymologies Tolkien gave the root ᴹ√SI “this, here, now” with derivatives like ᴹQ. sí or sin “now” and ᴹQ. sinya/N. sein “new” (Ety/SI). The root √SI was mentioned a couple times in Tolkien’s later writings, usually glossed “this” (PE17/67; VT48/25; VT49/18) and in one place with the variant √SIN (PE17/67). This root was not entirely without competition in Tolkien’s later notes, however: in one place he gave primitive ✶khĭn- as the possible basis for Q. hí “here” and S. hí “now” in 1968 notes on demonstratives, though it appeared beside primitive ✶si- forms (VT49/34 note #21).
Tolkien introduced the (unglossed) root ᴹ√THIN in The Etymologies of the 1930s with derivatives like ᴹQ. sinde/N. thinn “grey, pale” and ᴹQ. sinta-/N. †thinna- “fade” (Ety/THIN). In this document it was the basis for the name Ilk. Thingol; in earlier writings from the 1920s the name ᴱN. Thingol was based on the word ᴱN. thing “prince” (PE13/154). The root √THIN or √ΘIN was mentioned a couple times in Tolkien’s later writings as the basis for “grey” words (PE17/72; WJ/384). In a note from the mid-1960s Tolkien considered making the root √STIN the basis for “grey”, as a privative formation = √S-TIN = “without sparkling” (PE17/184), but I think this was a transient idea.