I am evaluating ERPNext and, like italjapan and InnovaERP, I need an ERD. No FKs? How do you ensure referential integrity in the database without FKs? Are you relying on code in each module/application to maintain table relationships?
In my experience, pushing as much logic as possible into the database through the use of stored procedures, triggers, FK constraints, and views (preferably materialized) will improve integrity, performance, and security.
Exploring the ERPNext database, I see 330+ tables and nothing else. ERPNext is a complex system. Why not use all the capabilities of MariaDB? I am a bit concerned about long term enhancement, maintenance, and support of an ERPNext implementation.