Not OP, but it seems that there is still a huge sentiment that Unity is not a "safe" platform to migrate to because of their relatively antagonistic approach to monetization guidelines compared to other open source game engines. I do think it makes sense to also consider Godot given his coworker is his brother who is stated to be new to game development, it has a scripting language even simpler than C#, more like python. Additionally, one might expect that someone more into Rust might prefer the C++ integration that Unreal offers. I think the timeline had an effect here too, as it's not been until recently that people have been taking Godot more seriously.

Personally, literally anything except Unity. The fact that they tried to retroactively change terms on developers means that it will be a long time before I feel comfortable trusting they won't try it again.

Maybe godot? The unity scandal recently is not great for developers.