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.
A word in The Etymologies of the 1930s glossed “line, row”, derived from primitive ᴹ✶tēñe under root TEÑ (Ety/TEƷ). It might replace N. tê “line, way” < ᴹ✶teʒē based on the earlier form of the root ᴹ√TEƷ (Ety/TEƷ), or the two words might co-exist: with tî the result of long ē becoming ī [✱tēñ(e) > tī(ʒ)] and tê retaining e because the e was not long primitively [✱teñē > teʒe > tē].
Conceptual Development: ᴱN. tî or tí “line” appeared in the Early Noldorin Grammar and Early Noldorin word-lists of the 1920s (PE13/121, 154). The Gnomish Lexicon of the 1910s had G. tî as an adjective meaning “straight” but also a noun meaning the same thing as either G. tion “a straight line” or G. tê “mark, line; track; path” (GL/69, 71).
Neo-Sindarin: For purposes of Neo-Sindarin, I would assume tê and tî coexist, with tê “line, way” used for a continuous line, but tî for a line or row of discrete items, as in [N.] tiwdi “alphabet, (lit.) ✱letter-row”. I would further assume they were based on the later root √TEG “line” (PE19/97), from primitive ✱tegē and ✱tēge respectively.