I have been trying to read dharmasUtras and smRRitis about the daily and fortnightly yajnas. These are very rarely performed these days. They are definitely obligatory on some people according to these texts, but there are various qualifications for it. What qualifications make the agnihotra and darshapUrNamAsa obligatory? Please try to find a comprehensive list from dharmasUtras and smRRitis, including by varNa and Ashrama, as well as others.
- The agnihotra is not obligatory unless a couple choose to kindle their fires. for more details see this answer.
- For a person who is performing agnihotra, there are other mandatory occasional yajnas to be performed. The darshapurnamasas are the two primary ones there. There is no special extra qualification for the darshapurNamasa. It can be considered as a specialized form of the daily agnihotra to be performed twice a month.
You can read more about this topic in David Knipe's Monograph "Vedic Voices".