One essential tradition regarding ‘studies in Early Buddhism’ is Samyutta/Samyukta Buddhism, which is based on Ven. Yinshun’s works.
See Choong Mun-keat, The Fundamental Teachings of Early Buddhism (2000) pp. 7-11, indicates that the Chinese scholar-monk Yinshun has demonstrated the historical important of Samyutta-Nikāya/Samyukta-āgama (SN/SA) in Early Buddhism in two books: The Formation of Early Buddhist Texts 原始佛教聖典之集成 (1971), and Combined Edition of Sūtra and Śāstra of Saṃyukta-āgama 雜阿含經論會編 (1983) (Cf. also pp. 2-7: “1. Historical background”).
In the following recent article, the same author provides further useful information on this topic/issue:
“Ācāriya Buddhaghosa and Master Yinshun 印順 on the Three-aṅga Structure of Early Buddhist Texts” in Research on the Saṃyukta-āgama (Dharma Drum Institute of Liberal Arts, Research Series 8; edited by Dhammadinnā), Taiwan: Dharma Drum Corporation, August 2020, pp. 883-932.
https://www.academia.edu/44055729/%C4%8 … hist_Texts
https://www.academia.edu/39352226/The_F … ukta_agama
According to Ven. Yinshun, Saṃyukta-āgama/Saṃyutta-nikāya was not, at first, being termed as nikāya or āgama, but generally named the ‘Connected Discourses’ 相應教 Saṃyukta-kathā. About the term Saṃyukta-kathā, see p. 899, note 21 in the above-mentioned paper (2020) by Choong Mun-keat.
Calling the Saṃyukta/Saṃyutta as āgama/nikāya ‘collection’ was until when the other three nikāyas/āgamas (MN/MA, DN/DA, AN/EA) were gradually developed and expanded from it (相應教 Saṃyukta-kathā). Cf. pp. 10-11 in Choong Mun-keat’s Fundamental Teachings of Early Buddhism (2000).
The extant SA and SN, and also other āgamas/nikāyas, are sectarian texts. One can seek an understanding of early Buddhist teachings by studying them comparatively (p. 11).
It is likely the term, nikāya, was first being used in Early Buddhism for the four āgamas/nikāyas.
See also Ven. Yinshun in his book (CSA vol. 1) states that Samyukta-agama is the foundation of both the four agamas/nikayas and Mahayana Madhyamaka and Yogācāra’s essential teachings:
"《雜阿含經》（即《相應阿含》，《相應部》），是佛教界早期結集的聖典，代表了釋尊在世時期的佛法實態。佛法是簡要的，平實中正的，以修行為主，依世間而覺悟世間，實現出世的理想——涅槃。在流傳世間的佛教聖典中，這是教法的根源，後來的部派分化，甚至大乘「中觀」與「瑜伽」的深義，都可以從本經而發見其淵源 。這應該是每一位修學佛法者所應該閱讀探究的聖典。
… 其實，四部阿含是先有《雜阿含》 ，九分教是先有「修多羅」，「祇夜」，「記說」（這三分也還是先後集出），二者互相關聯，同時發展而次第成立的。《中阿含經》（[A1]一九一）《大空經》，說到「正經，歌詠，記說」（《中部》一一二《空大經》所說相同），正是佛教初期三分教時代的明證。
《瑜伽論．攝事分》中，抉擇契經宗要的摩呾理迦，是《雜阿含經》的部分論義，也就是「所說」——「修多羅」部分的論義。「修多羅」分陰、處、因緣、聖道四大類，在《雜阿含經》的集成中，「修多羅」是最早的，正是如來教法的根本所在 。"
https://www.dhammawheel.com/viewtopic.php?f=29&p=655491&sid=19870b64d850112ba7ad953247594377#p655491