I've been doing something very similar, but based on extendible hashing algorithms and LSM trees. I've come to the conclusion that a DAG of constraints, triggers and validations on a flat KV + entity model is probably the ideal data structure for 99% of projects I've worked on. You can get the benefits of the Datomic-like history by... skipping compaction and including a TX instant next to all record/assertions. I've found SQLite, Postgres, the LSM papers, Bitcask, and many other papers to be very helpful in terms of inspiration.

Edit: I'm prototyping in Python and implementing in Rust with intent to create a C API and embedding a Scheme runtime for "server"-side query and constraint parsing.