What Psi Ontic interpretations of QM can be made relativistic? Is the Many Worlds interpretation the only one?
This feels like a rather general list question so my answer's fairly short, but no, there are other $\psi$-ontic interpretations that may be made relativistic.
It's obviously less intuitive starting from this approach, and since there isn't a lot of interest in many interpretations (and many are originally derived from the Schrödinger equation), it seems understandable that $\psi$-ontic interpretations often present conceptual issues in regards to relativity - although this is an issue many $\psi$-epistemic interpretations also share outside of those that are a simple tweak to SQM.
That said, Bohmian mechanics is fairly simple to make relativistic - you can essentially replace the SE in Bohm's derivation with the Klein-Gordon equation and you're half of the way there (Nikolić, 2012).
There are relativistic GRW models also, e.g. Tumulka (2006).
Stanford encyclopedia also suggests there are various relativistic modal interpretations; but since they largely reference books I don't have access to their cited sources to discuss.
So I hope that answers your question.