Here are two commentaries on the Dharmadhatuvibhaga's stanza defining the nature of phenomena. Go Lotsawa states how it is the pure mind-stream, the luminosity, and Rangjung Dorje shows how it is the same everywhere and explains the emptiness of the three svabhavas.
Furthermore, the defining characteristic of the nature of phenomena
Is suchness, which lacks any distinction
Between apprehender and apprehended,
Or [between] objects of designation and what designates them.
Go Lotsawa's commentary (Mining for Wisdom within Delusion", p 303):
Therefore, what is called "the nature of phenomena" is the continuum of the mind that is of one taste, just like the expanse of space, because all phenomena of samsara do not go beyond this nature either. With regard to this, some [say] that [the nature of phenomena] is either [suitable as] a nonimplicative negation [in the sense] of being the nonexistence of apprehender and apprehended or that it is suitable as an implicative negation [in the sense] of existing as this very nonexistence of nonduality. Though there are assertions of such [negations] being the nature of phenomena, here it is not like that because the commentary [by Vasubandhu] explains it to be nothing but the continuum of stainless mind. For the Mahayanasutralankara also says that it is the pure luminous mind:
Mind is held to be always luminous by nature,
Contaminated [only] by adventitious flaws.
Apart from the mind that is the nature of phenomena,
Another mind's luminosity in nature is not taught.
The Third Karmapa's commentary (p 229-230):
This is what was stated above: "The defining characteristic of the nature of phenomena is suchness, which lacks any distinction between apprehender and apprehended, or [between] objects of designation and what designates them." This is the perfect nature, for which numerous synonyms are given in all the sutras and tantras. Glorious Naropa says:
This very being empty is awareness, mind.
Also bodhicitta is just this.
The tathagata heart is nothing but this.
Great bliss is precisely this.
What is called "secret mantra" is just this.
The reality of valid cognition is exactly this.
The fourth empowerment is this.
Connate joy is nothing but this.
The paramitas are precisely this.
Unity is simply this.
Great Madhyamaka is solely this.
Vairocana is this.
Vajrasattva is simply this.
The sixth family is only this.
The buddha disposition is just this.
Many enumerations, such as these,
Which are stated in the sutras and tantras,
Are for the most part based on this.
As for the meaning of noble Nagarjuna's statement that all phenomena lack a nature, the nature of all phenomena is that they neither arise by nature nor cease by nature. For this reason, since they are not real as being permanent or extinct, coming or going, or one or different, they are free from reference points. Therefore, they are both "all phenomena" and "the lack of a nature." The enumerations [of this lack of nature] are "the lack of nature in terms of characteristics," "the lack qf nature in terms of arising," and "the ultimate lack of nature," which are taught in relation to the imaginary, {528} the dependent, and the perfect [natures], respectively. One should understand that all [kinds of] emptiness are also divisions [that are derived] from this.
As for extreme interpretations of annihilation and permanence, Taranatha rejects both (The Essence of Zhentong, p 19-20):
In the Laṅkāvatāra-sūtra others asked, “Aren’t the major and minor marks of the enlightened essence the same features as those of the soul of the extremists?” In response, it was explained [by the Buddha] that “These are not the same features because they are of emptiness.” So, it is said that this enlightened essence does not exist as real, and if these major and minor marks were to exist, then they would be from the system of the extremists. It is also said that like space, what is not established whatsoever is known as the “enlightened essence.”
However, the mere recognition of different kinds of emptiness as meaning the unreal and the non-existence of anything whatsoever without definition is the mental fault of fixating onto one’s own erroneous philosophical system. From the [Laṅkāvatāra-] sūtra it reads, “The reason why these are not the same features as the extremists is because of the manifestation of emptiness, not because these major and minor marks are not manifest.” So, the claim that the enlightened essence of the completely radiant major and minor marks is explained to be interpretive in meaning is reduced to a mere deception within the world of lies.
The claim that the [enlightened] essence is permanent as asserted within the system of the extremists is also reduced to a refutation within the Essence Sūtras. Moreover, it is not acceptable to assert that the meaning of permanence is the permanence of a continuum. This is because saṃsāra, the entire subject-object complex, is merely the permanence of a continuum. So, if the mere permanence of a continuum was sufficiently permanent, then all conditioned phenomena would have to be permanent.
Brünnholzl (In Praise of Dharmadhātu) has a section for the question "Is Buddha Nature an Eternal Soul or Sheer Emptiness?" There he quotes Mipham (p 105):
In this way, Buddhist and non-Buddhist philosophical systems cannot be distinguished through mere words, but as far as the profound essential point is concerned, they are as different as the earth is from the sky. Hence, after his arrival in Tibet, Atiśa said that, in the India of his days, it is difficult to distinguish Buddhist and non-Buddhist philosophical systems.
And concludes (p 109):
To summarize, when not just clinging to the words but understanding what is conveyed by these words, let alone Nāgārjuna’s Dharmadhātustava, in Indian Yogācāra texts too, there is no reifying interpretation of tathāgatagarbha. The teachings on buddha nature were never designed as a doctrinal or ontological alternative to or replacement of emptiness. Tathāgatagarbha—the luminous nature of the mind—is not regarded as a monistic absolute beside which all other phenomena have a mere status of emptiness. Rather, it is the natural state of our mind, in which no self-delusion is ever at work. The default example used throughout tathāgatagarbha texts for this nature of the mind being without reference points, inexpressible, and indemonstrable is space. Still, in order to clarify that the ungraspable expanse of the mind is not just a mere inert vacuum, but that this expanse is vivid sheer experience—the natural unity of expanse and wisdom—these texts also give many examples for the luminous aspect of mind’s nature and its boundless inseparable qualities.