Well said. You are correct; the Theravada literally teach the exact opposite of Nagarjuna. And, I agree with you; the middle way between eternalism (Sarvastivada) and nihilism (Nagarjuna) is Theravada.
Nagarjuna…rejects… that there actually are dharmas. …there cannot be such things… Not only are the person and other partite things devoid of intrinsic nature and so mere conceptual fictions, the same holds for dharmas as well. This is what it means to say that all things are empty.
Nagarjuna’s Middle Way, Mark Siderits, page 7
Madhyamaka, also known as śūnyavāda (the emptiness doctrine) and niḥsvabhāvavāda (the no svabhāva doctrine) refers to a tradition of Buddhist philosophy and practice founded by the Indian philosopher Nāgārjuna (c. 150 – c. 250 CE). The foundational text of the mādhyamaka tradition is Nāgārjuna’s Mūlamadhyamakakārikā (Root Verses on the Middle Way). More broadly, madhyamaka also refers to the ultimate nature of phenomena as well as the non-conceptual realization of ultimate reality that is experienced in meditation.
-Wikipedia page on Madhyamaka
Both samsara and nirvana,
Neither of these two exists;
The thorough understanding of cyclic existence
This is referred to as “nirvana”
-Nagarjuna, Sixty Stanzas, verse 6
There is no distinction whatsoever between samsara and nirvana; and there is no distinction whatsoever between nirvana and samsara.
-Nagarjuna, Mulamadhyamakakarika, chapter 25
It is the dhammas alone that possess ultimate reality: determinate existence “from their own side” (sarupato) independent of the minds conceptual processing of the data. Such a conception of the nature of the real seems to be already implicit in the Sutta Pitaka, particularly in the Buddha’s disquisitions on the aggregates, sense bases, elements, dependent arising, etc.,…
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Thus by examining the conventional realities with wisdom, we eventually arrive at the objective actualities that lie behind our conceptual constructs. It is these objective actualities – the dhammas, which maintain their intrinsic natures independent of the mind’s constructive functions……
…the commentaries consummate the dhamma theory by supplying the formal definition of dhammas as “things which bear their own intrinsic nature” (attano sabhavam dharenti ti dhamma).
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…concretely produced matter…possess intrinsic natures and are thus suitable for contemplation and comprehension by insight.
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Great seers who are free from craving declare that Nibbana is an
objective state which is deathless, absolutely endless, unconditioned,
and unsurpassed.
Thus as fourfold the Tathagatas reveal the ultimate realities—
consciousness, mental factors, matter, and Nibbana.
-Bhikkhu Bodhi, Acariya Anuruddha, A Comprehensive Manual of Abhidhamma, pages 3, 15, 26, 235, 260