A likely precursor to this root appeared in the Qenya Lexicon of the 1910s as ᴱ√‘(A)ṚM(A)R and ᴱ√‘ṚMṚ with a Gnomish form ᴱ√grimri· (QL/32), indicating the actual primitive form was ✱ᴱ√ƷṚMṚ. Derivatives of this early root include ᴱQ. arm- “gather, collect” and G. grim “host, folk”, the last of these the likely precursor to N. rhim.
The root ᴹ√RIM appeared in Primitive Quendian Structure: Final Consonants from 1936, glossed “host, large number” >> “number, plenty” (PE21/57). It also appeared in The Etymologies written around 1937, with gloss “abound” and derivatives such as ᴹQ. rimbe/N. rhim “crowd, host” (Ety/RIM). The roots ᴹ√SRĪ, SRĬMĬ, and SRIMBI “abound” appeared in Demonstrative, Relative, and Correlative Stems (DRC) from the late 1940s, quickly revised to ᴹ√RĪ, RĬMĬ, and RIMBI (PE23/100 note #34). Quenya and Sindarin forms Q. rimbë and S. rim continued to appear in Tolkien’s later writing (Let/382; PE17/50; UT/318), so it is likely the root √RIM remained valid, especially given the prevalence of suffix -rim in Sindarin collective names.
An unglossed root in The Etymologies of the 1930s with derivatives like ᴹQ. ríma “edge, hem, border” and N. rhîf “brink, brim” (Ety/RĪ). Tolkien then added a note “alter to SRI-” without revising the derivatives (EtyAC/RĪ). Given that all its derivatives indicate primitive rīm-, it is almost certainly a later iteration of the unglossed root ᴱ√RIMI from the Qenya Lexicon of the 1910s with derivatives like ᴱQ. rim- “border on, lie at edge, neighbour” and ᴱQ. rímen “border, shore”, given in the same entry with the root ᴱ√RIPI with which it was apparently confused (QL/80). ᴱ√RIMI likewise had derivatives in the contemporaneous Gnomish Lexicon such as G. rim “a stripe, line; border, fringe” (GL/65), though blending with ᴱ√RIPI complicates the analysis; see that entry for further details.