Last year, Postgres support was merged into the code base. Unfortunately it is not yet possible to use structured SQL with Postgres because compatibility with BBOLT’s key-value model needs to be retained. This task adds SQLite support to LND. SQLite is widely supported across (mobile) devices and has the potential to eventually replace BBOLT. At that point, the possibility of a gradual migration to structured SQL opens up.
An initial proof of concept which may be used for inspiration is referenced in the issue description. One of the main missing pieces is an abstraction layer that covers both Postgres and SQLite.