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”.
The root √TUL was used for Elvish words having to do with motion towards a speaker for much of Tolkien’s life, but the precise meaning evolved over time. The earliest appearance of this root was as ᴱ√TULU “fetch, bear, bring; move, come”, but with an original sense = “uphold, support, bear, carry” (QL/95). It had a derived verb ᴱQ. tulu- matching the verbal sense of the root along with an added sense “produce, bear fruit”, but it had other derivatives like ᴱQ. tulma “bier, tray” and ᴱQ. tulwe “tall thin pillar, standard, pole; banner” connected to the older “support” sense of the root. It had a similar divergence of meaning in its derivatives from the contemporaneous Gnomish Lexicon, such as G. taul “a pillar” vs. G. tul- “bring; come to” (GL/69, 71). This 1910s root ᴱ√TULU may also have been connected to ᴱ√TḶPḶ which likewise had derivatives having to do with “support” (QL/93).
In The Etymologies of the 1930s the root ᴹ√TUL had the gloss “come, approach, move towards (point of speaker)” with derivatives like ᴹQ. tul-/N. tol- “come” and ᴹQ. tulta-/N. toltha- “send for, fetch, summon” = “(orig.) make come” (Ety/TUL); in the 1930s the “support” words seem to have been transferred to (probably unrelated) ᴹ√TULUK. √TUL “come” was mentioned regularly in Tolkien’s later writings (PE22/103; PE17/188; PE22/156), in one 1969 note with the extra gloss “approach” (PE22/168), so the 1930s senses for the root seem to have been retained thereafter.