Historically, emptiness stems from anatta, not-self, so the oldest texts in regard to the topic would be the Suttas in the Pali Canon.

SN 22.59: Anattalakkhaṇasutta explains anatta in terms of the five aggregates.

MN 1: Mūlapariyāyasutta explains anatta in the elements, the perceived, the all, and Nibbana.

SN 35.85: Suññatalokasutta:

“It is, Ānanda, because it is empty of self and of what belongs to self that it is said, ‘Empty is the world.’ And what is empty of self and of what belongs to self? The eye, Ānanda, is empty of self and of what belongs to self. Forms are empty of self and of what belongs to self. Eye-consciousness is empty of self and of what belongs to self. Eye-contact is empty of self and of what belongs to self…. Whatever feeling arises with mind-contact as condition—whether pleasant or painful or neither-painful-nor-pleasant—that too is empty of self and of what belongs to self."

Venerable Nāgārjuna, the founder of the Madhyamaka school, expanded anatta into - all phenomena are empty of intrinsic existed. I found Nāgārjuna’s Middle Way a good entry point into Nāgārjuna's teachings.

I think all branches more or less agree on these two teachings, but if we must divide, Mahayana and Vajrayana lean more into the works of Venerable Nāgārjuna while Theravada the Pali Canon.

When discussing other traditions outside Theravāda or early Buddhism, the use of the word development can be somewhat acceptable. However, from a religious perspective, these teachings are not the result of development but of awakening.

Within Theravāda itself, the term Anatta is more commonly used. One well known Thai teacher has used the term Suññatā Abhidhamma, which clearly shows influence from Mahāyāna ideas of Śūnyatā.

Personally, I have conducted research and found that many people misunderstand this topic in ways that differ from what the Buddha actually taught. Relying on the Tipiṭaka alone is not sufficient. One needs to consult the commentaries in some depth to see clearly that the matter is not as commonly assumed. Fortunately, in Thailand the commentaries are available in digital form, which makes research much easier. Even so, most people remain attached to their existing views, and even when presented with evidence that aligns with the Buddha’s words, they often seem to overlook it.

I would like to share my work, but it is currently in Thai. Once it is finished and translated into English, I will share it.

Personally I don't think you'll find much benefit in exploring shunyata academically. The basic teaching is in the heart sutra, but it's not understandable as philosophical concept. The Buddha was teaching about meditation experience. All of these teachings are guidance for meditation.

The so-called first turning teachings outline the path of the shravaka, seeking freedom from suffering. The Mahayana teachings of the second turning are taking a new approach. At some point we begin to see that "me" escaping suffering doesn't quite work. "Me" -- egoic attachment -- IS suffering. So Mahayana stresses teachings to dissolve the sense of dualistic -- self/other -- reality. Chiefly that's the teaching on shunyata and the focus on taking bodhisattva vow -- giving up enlightenment for the sake of others. In other words, if we're going to give up egoic grasping then we eventually have to actually do that and not just try to control kleshas. You won't be there to enjoy your own enlightenment. There will be no "me" to rest on laurels or cash in on the end of suffering. We just have to surrender to nowness.

A general outline: Shunyata or emptiness is a Mahayana teaching. Theravada emptiness refers to pratityasamutpada. Same word in English, very different teaching. Shunyata is essentially pointing to the non-graspability of experience. Form and emptiness arise together. Things appear vividly, but the experience is ungraspable. That's a description of what the Buddha found to be the most basic nature of experience. It's not something to be intellectualized. It's something to experience. The shunyata teaching could be thought of as a signpost in that sense.

I'm not sure how much that can fit into comparative religion. If you intend to compare then you'll be assuming a context or paradigm by which you define the nature of religion. So you can't help bringing your own preconceptions to it. Christianity, for instance has no such concept. It takes a reverse approach. Instead of giving up me there's a surrender to God, which is All. Same difference, but a different approach and different logic.