Sounds like "Migrating away from Bevy towards Unity"; the Rust to C# transition is mostly a technical consequence.

Bevy: unstable, constantly regressing, with weird APIs here and there, in flux, so LLMs can't handle it well.

Unity: rock-solid, stable, well-known, featureful, LLMs know it well. You ought to choose it if you want to build the game, not hack on the engine, be its internal language C#, Haskell, or PHP. The language is downstream from the need to ship.