Written tær (with ae-ligature) in the Etymologies, rectified here according to VT/46:18 (which also lists the ligature)
Noldorin
tîr
adjective. straight, right
tîr
adjective. straight, right, right, *correct, [G.] upright, honest; esteem, regard, honour; [N.] straight
taer
adjective. straight
aran
noun. king (used of a lord or king of a specified region)
feir
noun. right (hand)
forgam
adjective. right-handed
forn
noun. right, north
fœir
noun. right (hand)
taer
adjective. straight
taur
noun. king (only used of the legitimate kings of whole tribes)
In LotR/IV:IV, Frodo is called Daur, which might be the mutated form of this word
âr
noun. king (used of a lord or king of a specified region)
âr
noun. king
The adjective taer “straight” appeared in the entry for the root ᴹ√TEÑ “line, direction” as a derivative of ᴹ✶teñrā (Ety/TEÑ). In The Etymologies as published in The Lost Road, Christopher Tolkien gave its form as tær (LR/392), but Carl Hostetter and Patrick Wynne corrected this tær, taer in their Addenda and Corrigenda to the Etymologies (VT46/18). The form taer may have replaced N. tîr “straight, right” from the earlier version of the entry {ᴹ√TEƷ} > ᴹ√TEÑ, where Tolkien derived N. tîr from tēra < ᴹ✶teñrā (Ety/TEƷ; EtcAC/TEƷ).
Conceptual Development: The Gnomish Lexicon of the 1910s had G. tîr whose archaic meaning was “†straight”, but whose later meaning was “upright, honest” or (as a noun) “esteem, regard, honour” possibly due to a blending with other roots (GL/71). Early Noldorin word-lists had ᴱN. tain “straight” derived from ᴱ✶tegnā (PE13/153, 165).
Neo-Sindarin: The best Neo-Sindarin forms for these words are unclear. 1930s N. taer seems to have an abnormal modification of the ancient vowel to ǣ so that ǣ > ai and then ai > ae. The expected Noldorin development of primitive ᴹ✶teñrā would be the vanishing of ñ (> ʒ) with lengthening the vowel to ē and then ē > ī, producing N. tîr as in the earlier version of the root’s entry.
Another complication is that in later writings, Tolkien changed the root to √TEG “line” (PE19/97), repurposing √TEÑ to mean “indicate, point to” (WJ/394; PE22/149). The expected result of primitive ✱tegrā is teira > tair; compare S. cail < ✶keglē. However taer might still be the Sindarin result if there was an abnormal change of the ancient e to similar-sounding ę̄ (the 1950s analog of 1930s ǣ) perhaps via a-fortification, so that [[s|[ę̄] became [ai]]] and then ae. This usage clashes with S. taer “lofty” from the 1950s (PE17/186), but I think the two adjectives could coexist: taer meaning “lofty” when applied to vertical things like mountains but “straight” when applied to (mostly) horizontal things like roads and lines.
I also think it is worth retaining N. tîr < ✱teñra < √TEÑ with an original meaning of “that which is indicated” (but no longer meaning “straight”), so that its modern Sindarin sense could be “right, ✱correct”. I would further assume tîr has its Gnomish meanings “upright, honest” when applied to persons, so that it means “esteem, regard, honour” when used as a noun.
If the conflict between taer “lofty” and taer “straight” bothers you too much, you might instead use tîr for both “straight” and “right, ✱correct” (analogous to ᴹQ. téra), though it is less clear where the sense “straight” comes from in Tolkien’s later paradigm of roots.