Of course there is D’Ambra's 1988 paper "Isometry groups of Lorentz manifolds", from which the theorem you state is taken.
A later paper, taking a more general perspective, is [this one][1] by D’Ambra and Gromov from 1991.

Zimmer's school had its impact then, with Kowalski's thesis from 1994 and further papers by Zimmer, Adams, Stuck, Witte-Morris and Iozzi.

Later contribution was made by Zeghib's school and in particular Charles Frances.

There are many other contributions. Take a look at the list of participants of [this conference][2].
Many of the later contributions are in the language of Cartan geometries or rigid geometric structures etc. so you might miss them by a careless literature briefing.

My advice it to look at some of Frances' late papers, such as [this][3], and the references there.
Maybe even better: make a contact with Frances who is actively working on the subject.


[1]: http://www.ihes.fr/~/gromov/PDF/1[74].pdf
[2]: http://cs.slu.edu/~scannell/lorentz/
[3]: https://arxiv.org/pdf/1804.08695.pdf