Flax seems neat but Stride3D exists. I mean realistically, Godot exists and Unreal exists. Going with something with even less support and even less foothold means that if you get a team you'll be working on training them up most of the time on the first, maybe first few titles. Flax doesn't seem to justify itself yet. Seems like it's closed licensing like Unreal, with overall higher costs than Unreal and fewer benefits. Godot is at least truly open source and free.
Also a reminder, Unity also had perpetual EULA clauses.