There are many open source alternatives in Clojure using this query language: https://github.com/simongray/clojure-graph-resources#datalog

'ish.

datahike would be the closest to datomic in terms of features/implementation (support for as-of, transactor etc).

Then in terms of maturity I think the choice is between xtdb and datascript, both are very solid/maintained but they are also vastly different.