Never had success decimating in Blender. It’s WAY too many vertices. Unusably huge.
Decimating ends up ruining the look of the model to get it down to a game ready size.
Even a simple house model, or humanoid, using Decimate in Blender there is no way that will be even recognizable.
Have you tried meshlab [1]? I've had pretty fantastic results reducing meshes from millions to some thousands.
Have you tried meshopt? It depends a lot on the art, view distance, and tuning, but it works well for my use cases.
(I agree with you about Blender though, I always hated Blender's simplify)