[√]ᴺ√STAL root. “*valiant”
√STAL root. “strong”
The unglossed root ᴹ√STALAG appeared in The Etymologies of the 1930s with derivatives like N. thala “stalwart, steady, firm” and N. thalion “hero, dauntless man” (Ety/STÁLAG), the latter a sobriquet of Húrin typically translated as “Steadfast” in the narratives themselves (S/199). Similar forms appeared in Early Noldorin Word-lists from the 1920s derived from the primitive form ᴱ✶stalga (PE13/153).
The root √STAL “strong” was mentioned in passing as the basis for the adjective Q. astalda in a rejected page associated with roots having to do with “large & small” words, probably from the late 1960s (PE17/115; VT47/26 note #26). The name Q. Astaldo “Valiant” appeared as a sobriquet of Tulkas in later versions of The Silmarillion (S/28), replacing the earlier name Q. Poldórëa of similar meaning (MR/146, 149; LR/206). For purposes of Neo-Eldarin, I would assume √STAL (and its derivatives) means “*valiant” rather than “strong”.
References ✧ PE17/115, 185
ᴹ√STALAG root. “*stalwart, steady, firm”
Reference ✧ Ety/STÁLAG ✧ STÁLAG