This root and ones like it were used for “high” things for much of Tolkien’s life. It first appeared as unglossed ᴱ√TAHA in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. tā “high; high above, high up”, ᴱQ. tahōra or tayóra “lofty”, and ᴱQ. tāri “queen”; it had a variant form ᴱ√TAʕA where the ʕ might be a malformed Y (QL/87). The corresponding forms in the contemporaneous Gnomish Lexicon were G. dâ “high” and G. dara “lofty” (GL/29), indicating the true form of the root was ᴱ√DAHA, since initial voiced stops were unvoiced (d- > t-) in Early Qenya (PE12/17). Primitive forms like ᴱ✶dagá > ᴱN. dâ/ᴱQ. tá “high” in Early Noldorin Word-lists of the 1920s indicates the root continued to begin with D for the following decade (PE13/141, 161).
In The Etymologies of the 1930s Tolkien gave this root as ᴹ√TĀ/TAƷ “high, lofty; noble” with derivatives like ᴹQ. tára “lofty, high”, ᴹQ. tári “queen” and N. taen “height, summit of high mountain” (Ety/TĀ). In Definitive Linguistic Notes (DLN) from 1959 Tolkien gave the root as √TAG or Tā- “high”, and in notes from around 1967 Tolkien gave √TAƷ as the explanation of the initial element of Q. Taniquetil and contrasted it with √TĂR “stand” (PE17/186). In 1970 green-ink revisions to the Outline of Phonology (OP2), Tolkien wrote a marginal note giving √TAƷ > TĀ “high”, but this note was rejected with a statement “transfer to Gen. Structure. No [ʒ] existed in Eldarin” (PE19/72-73 note #22).
This last rejection seems to be part of Tolkien’s general vacillation on the nature and phonetic evolution of velar spirants in Primitive Elvish in 1968-70. For purposes of Neo-Eldarin, I would assume the root form was √TAH or √TAƷ > √TĀ as the basis for “high” words, much like √MAH or √MAƷ > ✶mā was the basis for “hand” words.
The root √TAR appeared unglossed in the Quendi and Eldar essay of 1959-60 as the basis for Q. tára “tall, high” (WJ/417), a word that elsewhere was derived from √TĀ/TAƷ “high” (Ety/TĀ; PE17/186). The root √TAR was glossed “stand” along with derivative tāra “tall” in rough notes on the back of a discussion of the comparitive from around 1967 (PE17/186). The past tense for Q. tarne “stood” appears in other notes from this period, along with Q. astarindo, artarindo or astarmo “bystander” (PE17/70-71). √TAR “stand” seems to be a later iteration of ᴹ√THAR “stand” from the Quenya Verbal System (QVS) written in 1948; of this earlier root Tolkien said it “is only used [in describing the location of things] - except, of course, with reference to persons or animals when they are noted especially as ‘standing’ (not sitting or lying) - of mountains, high hills, towers, pillars” (PE22/126).