Which single jain text is completely dedicated to the concept of matter and offers the most thorough elaboration of that concept?

In Jain philosophy, no single universally accepted classic text is entirely and exclusively devoted only to the substance of matter (pudgala).

If you want the most focused and detailed traditional treatment of matter as a dravya, the closest and most authoritative works are:

• Dravyasaṅgraha — very concise, but clear on pudgala
• Tattvārthasūtra with its commentaries — systematic and authoritative
• Pañcāstikāyasāra — deeper philosophical discussion of the six substances, including pudgala

If your question is about karmic matter specifically, then Karma-grantha texts are the most relevant, because karma in Jainism is understood as subtle pudgala.

So the most accurate answer is:

There is no single Jain text that is completely dedicated only to matter, but _Pañcāstikāyasāra_ and the _Dravyasaṅgraha_ are among the most important texts for understanding pudgala in depth.