An adjective glossed “strong” and derived from the root √STAL in a page of notes having to do with “large & small” words, probably from the late 1960s (PE17/115). This page was rejected, but Tolkien used Astaldo “Valiant” as a sobriquet of Tulkas in later versions of The Silmarillion. As such, I would assume [ᴺQ.] astalda is valid for purposes of Neo-Quenya, but I would use it with the meaning “✱valiant” rather than “strong”.
Quenya
astaldo
masculine name. Valiant
astalda
adjective. strong, *valiant
Astaldo
the valiant
Astaldo noun "the Valiant", a title of Tulkas (Silm, MR:438); replaced Poldórëa.
astal
noun. valour
astalda
adjective. *valiant; strong
astallos
noun. azalea, rhododendron
Poldórëa
valiant
Poldórëa adj. "Valiant"; as title of Tulkas replaced by Astaldo (POL/POLOD, MR:146, 149. In GL:64, poldórëa is glossed "mighty", in QL:75, "muscular".)
Turcafinwë
strong, powerful (in body) finwë
Turcafinwë masc. name, "strong, powerful (in body) Finwë", masc. name; he was called Celegorm in Sindarin. Short Quenya name Turco. (PM:352), compare #turco "chief" (q.v.)
cánë
valour
cánë ("k")noun "valour" (KAN)
poldorë
strong, burly
poldorë noun? (not glossed, derived from polda "strong, burly": possibly "strength" as an abstract) (POL/POLOD)
The sobriquet of Tulkas (S/28), a masculinized form of the adjective astalda “strong” (PE17/115).
Conceptual Development: In the earliest Lost Tales, the sobriquet of Tulkas was ᴱQ. Poldórea (LT1/79), which was the adjective ᴱQ. poldórea “muscular” used as a name (QL/75). The name ᴹQ. Poldórea still appeared in Silmarillion drafts from the 1930s, variously glossed “Strong One” or “Valiant” (SM/79, LR/206). It appeared in The Etymologies as an adjectival form of ᴹQ. poldore “physical strength; might” from the root ᴹ√POL(OD) “physically strong” (Ety/POL).
In later writings, the name Q. Poldórëa appeared in Tolkien’s Words, Phrases and Passages from the Lord of the Rings from the 1950s, along with variants Poldor and Poldorno, where he considered a new meaning for this name as “breaker up of the hard/tough” with its second element coming from the root √DOR “hard” (PE17/181). Ultimately, though the name was changed to Astaldo in Silmarillion revisions from the 1950s-60s (MR/149).