Having read through OP and its responses regarding “systematic and structured approach to Buddhism, Theravada Buddhism, and/or particularly Early Buddhism based on EBTs,” it seems all of them narrowly just focused on what you think about the core teachings of Buddhism.
I consider this is not a complete understanding of Buddhism. It is because one should not overlook the symbiotic relationship between “Folk Buddhism” and “Essential Buddhism” from Early Buddhism and beyond.
This ‘symbiotic relationship’ in fact goes back to Early Buddhism; e.g. Samyukta/Samyutta Buddhism (SA/SN) was a system for both:
(1) the fundamental teachings of Early Buddhism (the so-called sutra-anga portion of SA/SN)
(2) early Buddhist adaptation of general Indian religious beliefs about divine beings (devas) (the so-called geya-anga portion of SA/SN).
Cf.:
The sūtra-mātṛkā (sūtra matrix, 契經, 摩呾理迦 or 本母) - Discussion - Discuss & Discover
Was Buddhism ever a “system”? - The Watercooler - Discuss & Discover
Early Buddhism resources Dhamma Wheel