Theism and Western atheism are on the same continuum. Both are realist.

[Angrboda]

I will not have time to catch up on this thread today.

If the Madhyamaka referenced by the thread starter is the same as the one I am thinking of, it is more than "a system of negations" and indeed forms one of the most influential schools in Buddhist philosophy, having important consequences for both Buddhist and non-Buddhist philosophy and psychology in East Asia. My understanding is that much of the Madhyamaka approach was developed by the Buddhist Nagarjuna, but I don't know a lot of the specifics. (I have a partial translation of The Transmission Of The Lamp, which details the biographies of various Eastern sages in the first millenia, which I studied at length once upon a time, so Madhyamaka is somewhat familiar to me, but I haven't truly systematically studied the school or its philosophies.)

For what it's worth, I wouldn't describe any Buddhist ontology as a form of realism (particularly given their general inclusion of the doctrines of Anatta and dependent origination), but it's hardly novel for Buddhists to attempt to crassly legislate their own usage of terms in order to extricate themselves from political or philosophical difficulties (see, for example, the history of Buddhists attempting to redefine Astika and Nastika to place themselves as an orthodox Hindu tradition, in distinction to the more common view of it as Nastika, or heterodox Hinduism).

That being said, addressing the OP alone, there are multiple avenues of defense or justification for the overall notions that the poster is referencing. I personally wouldn't consider a straight up-the-middle approach based on the middle path, as this appears to be, but whatever. (And ignoring the obvious implicit cosmological and prime mover arguments, which may not necessarily be authentic to Madhyamaka thought.) There are other arguments, beyond just this one. In addition, there are both nuanced and not so nuanced approaches to the question. The OP clearly falls on the "not so nuanced" end of the spectrum. (Incidentally, it's quite plausible that Siddhartha himself fell at the other end of the spectrum, in some respects, tending to eschew pointless speculation about the metaphysical notions of the nature of reincarnation and karma.) For my part, while I greatly admire Nagarjuna and Madhyamaka philosophy, I think other approaches are far more likely to yield a satisfactory defense. Specifically, the five skhandas --> Anatta --> dependent origination --> the OP's brand of realism. The only real point of contact between the notions the OP appears to reference and that of classical western realism, is that both are monistic philosophies, postulating a universe filled with basically only one kind of "stuff." This differs radically from other religions such as Christianity, which openly seem to embrace duallism.