Well LotR qualifies as Tier 1-A for three (maybe more) reasons:
- Tolkien uses the same language to differentiate Eru's and the Ainur's level of existence to Eä (the universe) as he does to differentiate between reality and mythology
- Eru holds all of Eä as mere thought, being able to maintain it in his mind. To Him, it is on the same level as fiction is, as an entire space-time is but merely thought in His mind (indeed, the language is used describes Eä as but one finite fictional sub-creation under Eru/God's infinite creative potential. Meanwhile Tolkien states both space and time in Eä being limitlessly extensive, aka infinite. In essence a 4-D Space-Time is finite and fractional compared to Eru)
- Eru is described as an author, with his interventions being that of an Author, as is his role, level of existence, etc.
The Ainur qualify for this for a few simple reasons.
- Eru regards them as "real" existences he can interact with compared to Eä which is but fiction to him
- They shake the Timeless Halls and Void with their Music, they can thus affect a realm of existence that is "primary" to Eä which is "secondary". The difference being between that of fiction and mythology to Tolkien
- They are sub-creators that inhabit a similar cosmological position to authors like Tolkien (within cosmology, Tolkien exists as the sub-creative force to the Legendarium).
Tier 0 is a confusing topic for me but from the looks of it, interaction with tier 0 isn't necessarily impossible. God from Dante's Work is considered tier 0 in Ultima's new system, and they have interaction with others to an extent. Eru is unchanging, unbound by hierarchy, is indivisible from himself, and is the singular omnipotent of the Legendarium.
I don't know 100% though, but
Ultima at least believes Eru is Tier 0. This might mean the Ainur upscale in some way (not Tier 0 obvs), but that's another topic and I'm no tier 1 expert.